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The English Mechanic

AND

MIRROR OF SCIENCE AND ART.

FRIDAY, MAY 20, 1870.

WHAT STRIPES THE SUNBEAM.

BY A FELLOW OF THE ROYAL ASTRONOMICAL
SOCIETY.

(Continued from page 173.)

EVERYBODY who has sealed a letter with a

lighted taper in a room into which the Sun was shining, must have noticed that the candle-flame cast a perfect shadow; so that such shadow, after all, is only a question of the relative intensity of two lights. We may hence see how it is possible that, although this line D may have some luminosity, yet, stopping out or absorbing a ray of the same refrangibility as itself (in front of which, as we may say, the refraction of the prism has placed it), the surrounding part of the spectrum is so incomparably more brilliant as to give the idea of a black line from the mere effect of contrast. Professor Roscoe, in one of the splendid lectures which he delivered in May, 1868, before the Society of Apothecaries in London, performed a striking experiment to illustrate this. He first produced a large sodium flame, and then in front of it placed a little one. Under these circumstances the smaller flame absorbed the yellow rays from the large one at the back, and looked absolutely dark and smoky when optically superposed on it. He subsequently vapourised other salts in the small front flame; but as the rays emitted were of a different refrangibility to those issuing from the great flame, they no longer absorbed any of its light, and the smoky appearance entirely vanished.

If we thoroughly comprehend the nature of these phenomena, we are now prepared to follow Kirchhoff to the remarkable conclusion which he draws from them. They are shortly these :It is probable that incandescent gases possess the power of especially absorbing rays of the same degree of refrangibility as those which they emit; that, therefore, their spectra can be reversed that is to say, that when light of sufficient brilliancy giving a continuous spectrum is passed through them, their bright lines are turned into dark ones, and appear on the continuous spectrum, stopping out all those rays whose refrangibility is identical with their own. The way in which this principle is applied to the Chemistry of the Sun and Stars need not now present much difficulty. We have said above that, in order to determine the position of the bright lines in the spectra of various substances, they were compared directly with the dark ones of the Solar spectrum. In mapping in this manner the spectrum of iron, Kirchhoff was extremely surprised to find that every bright iron line had its corresponding dark solar one! Just as we had seen that the Sodium line and the Solar line D were coincident, so with each of the iron lines, of which 460 have been mapped. Nay, further, not only had each of these 460 bright lines its dark counterpart in position in the Solar spectrum, but they agreed absolutely in breadth and depth of shade, the brightest in the iron spectrum being the darkest in the Sun's light, and rice versa, a resemblance, or rather identity, extending to the minutest particulars.

It would be worse than childish to regard this as mere chance. Two lines might agree fortuitously; four would be in a much higher degree unlikely to coincide casually, but when it comes to the perfect and entire agreement of 460, it is absolutely certain that the lines must have a common origin. Let us see now, in the light of the knowledge we have acquired, what such origin is. Obviously, the light which forms the Solar spectrum must have passed through the vapour of iron, and must have suffered the absorption which that vapour exerts. Where can that vapour be? It may be in our own atmosphere, or in that of the Sun. That it is not in the Earth's atmosphere is rendered probable by the following considerations. Imprimis, it is not very easy to understand how our own atmosphere could contain a sufficient amount of iron vapour to yield such exceedingly sharp and distinct ab

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sorption lines as are visible in the Solar spectrum;
and, in the next place, the lines remain unaltered
when the Sun is close to the horizon, and when
we consequently view it through a greatly-in-
creased thickness of air. On the other hand, the
very high temperature which it is only reasonable
to assume must subsist in the Solar envelope
would render it likely enough that such vapours
were present in it. "Hence," says Kirchhoff,
"the observations of the Solar spectrum appear to
me to prove the presence of iron vapours in the
solar atmosphere with as great a degree of cer-
tainty as we can attain in any question of natural
science."

explained? Kirchhoff's idea, propounded in his book, is that the Sun consists of a solid or liquid nucleus heated to the brightest whiteness, and giving a continuous spectrum, and surrounded by an atmosphere of somewhat lower temperature, containing the vapours of the various substances, which, by stopping off the rays corresponding in refrangibility to those of their own bright spectra, filter, as it were, the solar light on its way to us, so that its spectrum appears to us crossed with dark lines. On page 200 of our last volume, we have given a figure of the total eclipse of the Sun as seen at Guntoor, in India, on August 18, 1868, and have there very briefly of one terrestrial element in the solar atmosphere masses of red flame (which have been at once He goes on to add: "As soon as the presence referred to the discovery that the wonderful was thus determined, and thereby the existence the greatest admiration of, and puzzle to, former of a large number of Fraunhofer's lines ex- observers), visible when the Sun's disc was finally plained, it seemed reasonable to suppose that obscured, consist of stupendous flames of incanother terrestrial bodies occur there, and that by descent hydrogen gas. Prior to the date of which exerting their absorptive power they may cause we speak, these astonishing objects had never the production of other Fraunhöfer lines. For it been perceptible, save under the circumstances of is very probable that elementary bodies which a total eclipse, but, finding that they yielded a occur in large quantities on the earth, and are bright line spectrum, it struck M. Janssen (of likewise distinguished by special bright lines on whom we have previously spoken, and who went their spectra, will, like iron, be visible in the solar out to Guntoor as the French Government astroatmosphere. This is found to be the case with nomer) that they or rather their spectra, might calcium, magnesium, and sodium. The number be observed at a time when the Sun was unobof the bright lines in the spectrum of each of scured--an observation in which he was successful these metals is indeed small, but those lines, as on the very next day. It happened, very well as the dark ones in the solar spectrum with curiously, that Mr. J. Norman Lockyer, F.R.S., which they coincide, are so uncommonly distinct arrived, quite independently, at the same en that the coincidence can be observed with very viction as M. Janssen, and, without any commugreat accuracy. In addition to this, the circum-nication with him, also succeeded in making out stance that these lines occur in groups renders the the spectrum of these prominences, Mr. Lockyer observation of the coincidence of these spectra who finds that these gigantic uprushes of hy-' more exact than is the case of those composed of drogen are only local aggregations of an envelope single lines. The lines produced by chromium surrounding the Sun, proposes to call this enform also a very characteristic group, which, like- velope the chromosphere, to distinguish it from the wise, coincides with a remarkable group of Fraun- absorbing atmosphere on the one hand, and from höfer's lines; hence I believe that I am justified the white-hot photosphere on the other. Into this in affirming the existence of chromium in the part of the subject, however, it is not very imsolar atmosphere. . . . Barium, copper, and zinc portant that we should enter into much detail. appear to be present in the Solar atmosphere, but Research is still being sedulously carried on, only in small quantities; the brightest of the and the time has scarcely arrived for anything lines of these metals correspond to distinct lines more than a hypothesis which shall account in the Solar spectrum, but the weaker lines are generally for observed appearances. We will not noticeable. The remaining metals which I turn, then, to the results of spectroscopic rehave examined-viz., gold, silver, mercury, alu- search as applied to the fixed stars and nebulæ. minium, cadmium, tin, lead, antimony, arsenic, stronlium, and lithium, are, according to my observations, not visible in the solar atmosphere." The following are all the metals which are at present known to exist in the Sun :-1. Sodium; 2. Calcium; 3. Barium; 4. Magnesium; 5. Iron; 6. Chromium; 7. Nickel; 8. Copper; 9. Zinc; 10. Strontium; 11. Cadmium; 12. Cobalt; 13. Hydrogen; 14. Manganese; 15. Aluminium; 16. Titanium.

Before, however, we do so, it may be as well to revert to the instrument by means of which our observations must be made, and to say a few words as to the form of it which is adapted for celestial observations. The pattern adopted by our most eminent living observer in this branch of science, Mr. William Huggins, F.R.S., it is needless to illustrate here, as it is only applicable to a telescope of very large size, and as we propose to describe more at length a form of It must not, however, be supposed that all the spectroscope with the practical working of which dark lines visible in the solar spectrum are we are ourselves more familiar, and which is inreferable to vapourised elements in his atmo-finitely better suited to the possessors of moderate sphere. Some of them, undoubtedly, have their telescopes than the costly and complicated one origin in our own, The late Sir David Brewster we have alluded to. It must suffice, then, to say was the first to point this out. M. Janssen, the that Mr. Huggins has two prisms (to increase famous French physicist, whom we shall have the dispersion) in his instrument, and that, by occasion to refer to again further on, experimented a very delicate adjustment, he gets and retains a on light passed through a column of high-pressure star between the jaws of the slit of it. The steam, and found that the steam exerted a strongly-star, being a point, will be lengthened into a mere absorbing power, groups of dark lines appearing line of light, so has to be spread out by a cylin between the extreme red and the line D. As the additional lines seen is the Solar spectrum when the Sun is close to the horizon coincide with those thus produced by M. Janssen, they must evidently have been produced by the absorptive action of the aqueous vapour of our own air.

And now what must we assume to be the physical constitution of the Sun, in order that the appearances and effects detailed may be best

drical lens into a band. There is the usual provision of a diagonal prism over half the slit for the direct comparison of any particular spectrum with that of a star, planet, or nebula. Premising that a spectroscope is worse than useless with a telescope of less than 4in. aperture, we will proceed to describe the form of which we have spoken, and which, from personal experience, we consider the most suitable for an

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Que form of chromate

a doubt, that one of the humours of my eye, aud | This consitutes
of the eyes of my fellows is a coloured medium, pseudopsis. Another form in which the more
probably some modification of blue. I suppose strongly marked complementary colours are mis-
it must be the vitreous humour, otherwise, I taken, Professor Kelland supposes to arise by the
apprehend, it might be discovered by inspection, facility possessed by the retina for starting into
which has not been done."* As Sir David vibrations of every period, when excited by those
Brewster has said in a paper on "Colour Blind- of one only, "just as a musical pipe or string
ness," that appeared in the " Philosophical Ma- gives all the notes of the chord at the same time,
gazine," of 1844, this hypothesis of Dr. Dalton is or as the different strings of a harp vibrate when
the only one capable of being proved or disproved. only one has been struck." Professor Wartmann,
Let us see to what result the attempt to prove it in a second paper, suggested a theory nearly
has led.
similar to that of Professor Kelland, in which he
says, "I admit that the seat of colour blindness
is in the retina, and I think it is produced by an
abnormal state of this nervous expansion, of
such a sort that it reacts similarly, under two or
more different coloured radiations. If the vibra-
tion caused by a red ray is identical with that
which a green produces, there will be confusion
of these colours. This theory is independent of
every system destined to explain light." Whether
the reader may chose to adopt either of these
theories or not, it seems generally evident that
the cerebro-retinal apparatus of vision in the
colour-blind is either through congenital defect,
or subsequent morbid change, unendowed with
that sensitiveness to colorific impressions, which
it possesses in those whose vision is normal.

Mr. Joseph A. Ransome examined Dalton's eyes after he was dead, and declares that the aqueous humour was found to be perfectly pellucid and free from colour. The vitreous humour and its envelope were also perfectly colourless. The crystalline lens was slightly amber-coloured, as usual in persons of advanced age. The tunics retina, choroid, and sclerotic, with their subdivisions, presented no peculiarity. And further, after the posterior portion of the outer coats had been removed, objects of different colours, particularly scarlet and green, both by transmitted and reflected light, were examined without any appreciable difference. Dalton's theory during his life did not gain many supporters, and after his death it may be regarded as completely disposed of. With reference to this hypothesis of Dalton, Wartmann has remarked that if the passage of the luminous rays through a blue medium, sufficed to produce colour blindness, the habitual use of blue glasses for spectacles, would have long ago confirmed this hypothesis, against which it forms, on the contrary, a very strong argument. Perhaps this argument is not of any great value, as colour blindness in one of its many forms, must certainly result from a blue vitreous humour, but we can coincide in the remark with which Wartmann dismisses the theory, that the opinion of Dalton cannot be adopted.

amateur. It admits of no measurement of the spectra, but this, as will be found, can only be effected by instruments of great size and power. It is the invention of Mr. John Browning, F.R.A.S., who read a short paper on it before the Royal Astronomical Society on June 11, 1869. It is shown in section in Fig. 8, and our description shall be in Mr. Browning's own words :"In the diagram A is a compound direct vision prism consisting of five prisms. B is an achromatic lens which focuses on the slit C, by means of a sliding tube, H; both the prisms and lens are fastened in this tube. K is a small rightangled prism covering half the slit, by the aid of which light may be seen reflected through the circular aperture in front of it. In this manner a comparison may be made with the spectra of metals or gases. The reflecting prism, with the ring to which it is attached, can be instantly removed and the whole length of the slit used if desired. DD is a ring milled on the edge; on turning this round both edges of the slit are made to recede from each other equally, being acted on by two hollow eccentrics. The lines can thus be increased in breadth without their centres being changed-a point of importance. E is a cylindrical lens attached to the tabe F, which slides into another tube, G. To use the spectroscope, the adapter L, which is the same size as If any of our readers have discovered that the Society's thread, has only to be screwed into their eyes possess the peculiarity of which this the eye-drawer of the telescope in the place of paper treats of, they may desire to know whether the ordinary Huyghenian eyepiece. The drawer their case admits of cure or palliation. We remust then be adjusted so that the slit G comes gret not to be able to offer any very great hope exactly to the foens of the object glass. This respecting an ultimate and complete cure, when should be tried beforehand by getting an image the colour-blindness has been congenital, but of the Sun, and then making a mark on the there are a certain class of diseases which for a drawer-tube of the telescope. When this has time cause the sufferer to experience all the anbeen done the tube can be set by this mark, and noyance and confusion that congenital colourthe spectroscope screwed into it at any time blindness is calculated to produce, and differs without any trouble in adjustment. If the cylinfrom it only in the respect that appropriate treatdrical lens be removed the spectrum of a star will ment may cause it to disappear. In the case of be a mere line of light. The cylindrical lens is colour-blindness, that was occasioned to a gentlefor the purpose of widening this line to such an Sir David Brewster, in a paper that appeared man by a fall from his horse, that we quoted in extent that the lines in the spectrum may readily in the "Philosophical Magazine," attempted to the earlier part of this article, the peculiarity be discerned. For this purpose the lens must be account for the peculiarity of colour blindness, by has irremoveably attached itself, and does not placed with its axis at right angles with the slit, a supposition very nearly akin to that of Dr. apparently admit of cure. Dr. Erle (himself a and the best distance from the slit is between Dalton, but he rejected the supposition of a member of a family in which seventeen cases of three and six inches. The nearer it is brought coloured vitreous humour, and substituted a this affection of vision have occurred), holds an to the slit the broader will be the spectrum; but coloured retina. We give his conjecture in his opinion which we believe is not generally supit should not be used too close, on account of own words :-"During the dissection of many porled by medical men, or observed fact, that " the diminution of the light. When it is desired hundred eyes, I observed in several cases that the succession of years modifies the Daltonism of to obtain the spectra of planets, comets, and retina had a marked French grey or pale blue the same individual," though he does not hazard nebulæ, or, indeed, any heavenly bodies possessing tint, which decidedly absorbed red light. I knew an opinion as to what extent advancing years considerable diameter in the telescope, the cylin- that in cases of colour blindness the vitreous induce a change. Dalton was as colour-blind at drical lens may be dispensed with advanta-humour was not blue, or even greenish blue, as the Oxford meeting of the British Association, geously." Dr. Dalton conjectured, but I could not assert when he compared the colour of his D.C.L. gown that in the same cases the retina might not be to that of the leaves of the tree, as in 1792, blue, and hence I was led to hazard the idea of a when he first discovered his colour-blindness, and blue retina, as one which might be admissible as as he remained to the day of his death in 1844. a cause of colour blindness, but only on the sup- Moreover, there is not on record one case where position that the choroid coat should prove to be a cure of congenital colour-blindness has been the seat of vision. The italics are Sir David effected, though nearly all would make some atBrewster's. I do not think that Sir David tempt to cure themselves of this defect, and, as Brewster ever attempted to extend this supposi-a rule, the more observant and the better edu tion into a full conclusion, and even granting that the choroid coat should perform the office usually ascribed to the expansion of the optic nerve, it seems doubtful whether a coloured retina could produce such an effect as that witnessed by the colour-blind.

Let us now see what the application of the instrument to the more remote bodies which spangle the firmament, has revealed.

(To be continued.)

COLOUR AND COLOUR BLINDNESS
BY "OMICRON."

Many eminent authorities, among whom we may quote Sir John Herschel, are of opinion that this false perception of colours, arises from a defect of the sensorium, by which it is rendered incapable of appreciating exactly those differences between rays on which their colour depends, and though such an opinion does not advance very far in the explanation, it has been almost universally embraced.

(Concluded from page 170.) PR OROFESSOR WARTMANN notices that there is prevalent an opinion, that the number of Daltonians who have blue eyes, is greater than the number possessing black eyes, but this opinion he confidently denies, and M. Seebeck entirely coincides with him. In the number of Daltonians examined by him, the majority bad black eyes, and in reckoning up all the cases of colour-blindness in which the tint of the iris has been noticed, he finds that the two colours are in equal numbers. Prof. Wartmann noticed, how-theory or suggestion to account for Chroma The Rev. Philip Kelland has published a ever, without laying any great weight on the Pseudopsis, which deserves to be mentioned. circumstance, that Daltonians "whose eyes are brown of the colour which English call hazel (noisette) present under an incidence more or ess oblique, a golden lustre of a peculiar

tint."*

may

period.

He regards the sensation of colour as due to the reception by the retina of pulsations of a given period, and though the manner in which the retina performs its functions is not precisely known, it be presumed to vibrate agreeably to their Dalton accounted for the peculiarities of his In the normal state, the retina is supposed to eye on what may be termed the "Chromatic be capable of responding to pulsations of every Theory." He noticed "that a sky-blue trans- period within certain limits, and of conveying to parent liquid modified the light of a candle, so as the mind the sensations which we term colour to make it similar to daylight, and of course re-appropriate to each period, but in other states of stored to pink its proper colour by day-namely, the retina this appears incapable of being effected, light blue." This induced him to publish his but a vibration occurs more readily in accordance theory thus: "It appears, therefore, almost beyond with one period of pulsation than with another.

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cated the sufferer is, the more decided is he that all attempts to cure him will prove a failure. Since an absolute cure is impossible, the possibility of remedying it by any mechanical means, is a question of the highest importance. Professor Wartmann says that there "exists a very easy means of rectifying, to a certain extent, the error of the appellation of colour," and his suggestion is to examine coloured objects through a transparent medium, "as a glass or a liquid of a certain known tint." Suppose this tint red, the impression of a green body and of a red body, the same at first to a naked eye, will become manifestly distinguished by the use of the transparent screen." Unhappily the tint of the coloured glasses cannot be prescribed in advance, the impossibility of a rigorous classification of the innumerable varieties of Daltonism obliges us to chose them a posteriori for each particular case. Perhaps the best means (and which in some cases answers very well) to convince a colour-blind person of his errors, is to make him examine the object both by daylight and artificial light, when often a very great distinction will be evident between the appearance of the object examined, but although the colour is often different under the two lights, it is possible that neither impression is the same as reaches a normal eye; for instance, Dalton says, "Pink appears by daylight to be sky-blue, a little faded; by candlelight it assumes an orange or yellowish appearance, which forms a strong contrast to blue." As a method of palliation, Dr. Wilson suggested employing glass, stained yellow or orange, by means of oxide of silver, and in

AND APPLICATION.*

BY J. T. SPRAGUE.

(Continued from page 148.)

other absorbing the hydrogen at the negative

to the degree in which this negative pole and liquid can be kept in their normal condition, or, at least, in an unchanged relation to conductivity and chemical action. Hence they all require a

119.As a suitable appendage to the simple separating medium, as to which a few practical

observations will be of value.

many cases he mentions these glasses proved of ELECTRICITY-ITS THEORY, SOURCES, pole, and the success is greater just in proportion great service to his colour-blind acquaintance. Sach experiments as we ourselves have been able to make on these means, as a palliative to-colour blindness, have not convinced us of any utility that they are to the general colour-blind public. Our investigations have led us to conclude that the media employed, to be of any use, must be selected by experiment by the sufferer himself. Let him choose several coloured glasses, and examine objects, the true colour of which he has been told, and when he finds that the employment of a particular glass most generally restores the proper colour to the object, that coloured glass must be retained for general use, though our experience has led us to conclude that a glass of a particular colour is only useful in examining objects that have the same colour, and of course such a plan as carrying varied coloured media is only useful to those who are partially colouredblind, to endeavour to detect the colour, and not the particular shade or tint, that the colour presents to a normal eye.

THE END.

THE INSTITUTION OF CIVIL ENGINEERS.

One

battery described on page 147, a mode of economising in the article of zinc deserves attention. Smee proposed what he called an "odds-and-ends" cell, composed of a jar in which a quantity of mercury was placed with scraps of zinc, broken plates, even raw spelter might be used by floating them in the mercury; a plate of platinised silver was then suspended in the jar and the acid solution added. This has been tried by many, but for many reasons has never given satisfaction, but I have devised a modification which has all its advantages without its evils. I take a cylindrical vessel, such as an old porous jar, and pierce its walls with holes, or, still better, make one of gutta percha, which is stronger; the lowest inch or so is not perforated, as it is to contain mercury, to the bottom of which is plunged a stout copper wire, amalgamated at its end, but covered everywhere else with gutta percha, and cemented to the side of the vessel, reaching to its top, where it is to be

WE have just received a report of the speechos
at the annual dinner of the Institution of
Civil Engineers. One would have thought that
at such a dinner the chief people present, and the
chief speakers, would be engineers, or, at all events,
that the chief topic would be Science. Nothing
of the kind. We find, from the report, that the
speakers were the President of the Institution,
Sir Edward Belcher, Earl Granville, Earl of
Derby, the Solicitor-General, Lord Chief Justice
Bovill, Mr. G. P. Bidder, the Archbishop of
York, Mr. Fowler, Sir Francis Head, Professor
Tyndall, Mr. Beresford Hope, and the Right Hon.
H. A. Bruce. The morning papers, as a matter
of course, gave a report of the proceedings, which
consisted of a report of the speeches of Lord
Granville, Lord Derby, and the Archbishop of
York. But scarcely any allusion was made
in these reports to the Engineers, or to Science at
all-or, in other words, Science, at a meeting of
scientific men, was only incidentally alluded to.
Now, who is to be thanked or blamed for this?
Nobody but the Engineers themselves.
would have thought that at an annual meeting
of one of our principal scientific bodies the soldered to a binding screw which is the zinc con-
chief men connected with that body would have nection. All the rest of the surface is pierced
had something to tell the world worth listening with as many holes as possible consistently with
to about the progress of Science. Instead of this strength, to allow free circulation of liquid. It
we have men quite able, we admit, to instruct the is then filled up with pieces of zinc, amalgamated
public on matters legal, social, or political, but and in as close contact as may be; the whole acts
altogether inadequate to say anything worth
listening to on scientific matters. We should and is used just as if it were an ordinary plate.
have thought that the Institution of Civil En-
gineers would have been the very last society that
would have permitted itself to be invaded by
platform, or bar, or House of Commons talk. We
could imagine smaller and weaker societies, that
like to bask in the sunshine of patronage, adopt-
ing such a policy; but the Civil Engineers can
stand alone; they are a numerous, sturdy,
aggressive body of men, and some of them are
quite able to talk as well as act. It may be all
very well to invite a few guests representing the
Army and Navy, the Bar, the House of Lords,
and the Commons, to respond to toasts in a festive
way; but to record verbatim their commonplace
talk about nothing in particular, and then point
to it as a report of an annual meeting of the
Institution of Civil Engineers, is simply ridi-
culous.

ENGLISH MECHANIC SOCIETY OF ARTS,

EDINBURGH.

The mercury is subject to little waste, but now
and then the whole should be removed, well
shaken up together, and repacked, and at times the
mercury as it becomes charged with metals should
be filtered by squeezing through a chamois
leather, the residue being preserved, and when a
quantity is collected, distilled to recover the
mercury, as should be done with all the brushings
off plates, &c., of amalgamated zinc. This
may be used either in such a battery as my
arrangement, or in a Daniell or other cell,
within a porous jar.

described so far have two faults. 1. Weakness 120. TWO FLUID CELLS.-All the combinations -Owing to the force being largely absorbed by the escaping hydrogen, hence their electro-motive force is low, and any large resistance greatly reduces the current they can yield. 2. Want of THIS society held its first monthly meeting on Constancy-As shown by the rapid fall in the Thursday, the 5th inst, Mr. C. E. Gordon presiding. experiments, though in my own form this is in having been read and approved of, a copy of rules was negative surface. For many purposes, constancy The minutes of preliminary and committee meetings great degree overcome by largely increasing the submitted, which, after a long discussion, were remitted is essential, and it is desirable in all, hence conagain to the committee. Office bearers were next tinual efforts have been made to overcome these appointed, when the following gentlemen were elected. two defects, and with considerable success, the Passmore Edwards, Esq. Editor and Proprietor of though a really constant battery has yet to be ENGLISH MECHANIC AND MIRROR OF SCIENCE, Honorary President: Mr. W. T. Williams, President: discovered, notwithstanding the praises bestowed Mr. C. E. Gordon, Vice-President; Mr. R. Marshall, by manufacturers and patentees on several forms. Secretary; Mr. Bennett, Treasurer. hibited (with description) of boomerang, &c., &c. Specimens ex-As yet constancy is only to be obtained by the The business for next meeting was announced to use of two agents, one acting on the zinc, the be the President's inaugural address. ing then adjourned till 2nd prox.

The meet

"Sigma."

121. POROUS JARS.-At first, animal membranes, bladders, ox gullets, &c., were employed, and they answer very well. In some cases good paper is useful. In many experiments requiring great resistance, glass tubes plugged with plaster of Paris, or even clay, are employed; for small experiments, or even for platinising silver, the bowl of a tobacco pipe may be used. For all practical purposes, however, unglazed earthenware is the only material of any service, and it is now to be obtained in any form and size, though its prices are unreasonably high. There are many qualities, and they must be adapted to the special purpose. These porous jars act only by the liquid they absorb, and as they very greatly reduce the area of liquid through which the action takes place, of course they greatly increase the internal resistance, and diminish the action; on the other hand, there is no possibility of preventing two solutions thus separated from mixing, and this causes waste by local action, as well as affects the regularity of the actions. Hence for long sustained action a thick and closegrained jar must be used, while an open and more porous one suits best for short periods and strong action. The most porous ones are of a red colour and soft material, the finest and most enduring are close-grained and white, and the best are of French make. There is a cream and orange coloured ware, some of which is very good. The best test is to fill them with water and see how long it is before it forms a dew on the outer surface, if it runs off the jar is not fit for use. It is a great improvement to render the bottom, and still more the part which is to remain above the liquid, non-absorbent, which is readily done by warming and applying carefully sufficient melt-d paraffin, taking care it does not extend too far if it does so by accident it is readily driven off by heat. If this is not done the salts rise up, efloresce, crystallise, and disintegrate the jar. For the same reason jars taken out of the liquids must not be permitted to dry, but should be kept soaking in water to prevent their destruction. This is of particular importance with jars used for the Daniell's cell, as they are very apt to get nodules of copper deposited on them wherever the zinc has touched the inner surface, and particularly at the bottom, where drops of mercury or flakes of zinc fall, and then the cell is very soon rendered worthless; if this occurs, the spot of metal should have some cement or gutta percha laid over it, so as to render it non-conducting.

122. THE DANIELL'S CELL.-This, the first

devised improvement, is also the most successful attempt to obtain constancy. To it also we owe the discovery of the electrotype process, and all it has grown into. Its principle is, that copper plate as the negative metal, is surrounded by a solution of a salt of copper, which is reduced; instead of hydrogen, copper is set free, and it is deposited on the negative surface, which is thus kept constantly renewed. The acid of the salt is transferred by electrolysis to the positive metal, through the porous medium, hence if a fresh supply of the salt is added to the solution to replace that removed, in the same condition; but still absolute conthis part of the arrangement remains constantly stancy cannot be obtained, because as the zinc dissolves, the solution belonging to it becomes back to this cell is that the copper salt passes by less active and less conducting. The great drawendosmose into the zinc solution, and acts on the zinc where the copper is deposited, and causes great waste by setting up local actions.

This defect may be to a great extent remedied by arranging a piece of wire gauze close to, but not touching the porous diaphragm, on the zinc is connected by a wire to the copper plate during side, in such a manner that any fluid passing towards the zine must go through the gauze; this action, it thus becomes electrically part of the negative surface, and in some cases will act as a single fluid cell, to some extent contributing to the action; at all events any copper salt which comes in contact with it will be reduced there instead of at the zine; when the battery is not acting the gauze should be connected to the zine by a fine wire of iron or brass, so as to keep up a very slight action for the same purpose.

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cylinder within the reservoir is of course perforated. Modes of construction may be varied to any extent. Thus instead of a copper containing vessel, a glass or earthern jar may be used with a cylinder of sheet copper, or such a jar may be covered inside with a film of wax, blackleaded, and the deposited copper will form its own surface, but the first is the best plan, especially as the sulphate of copper has a great tendency to climb up glass surfaces, on which it crystallises and finds its way by degrees to the outside.

123. But instead of cylinders flat plates may be used in a vessel across which a plate of porous material is fixed, and this form has several advantages, among others it is easy to make the cell itself serve as a depositing vessel by using models, seals, &c., in fact any objects we wish to copy, as the negative surface, by suspending these to a rod which forms the + pole of the battery. In this case the cell is best made of wood, lined with a resinous cement; gutta percha may be used, but has the disadvantage of facilitating the creeping process of acids and salts, which is troublesome and messy, besides causing loss of power by establishing paths by which the force escapes. I find resin melted with a little gutta-percha, paraffin, and a small quantity of boiled oil answer perfectly; the wood should be perfectly dry and warm when it is applied. Such an apparatus, which also serves for the single cell electrotype process is shown in Fig. 38; a

FIG. 38 +

b

is the box divided by the porous partition d; c is a place for holding the crystals; e and fare two bars of metal, to which are hung the objects to be copied and the zinc plate, and each is fitted with the necessary binding screw. The bars being moveable, it is easy to regulate the distances, and so to control the action.

Many of the telegraph lines are worked by Daniell's cells of this sort, that is to say fitted with plates in cells, either divided by a fixed plate or the zinc contained in a flat cell, only the side of which facing the copper is left porous, the others being smeared with tallow, &c., to diminish endosmose.

(To be continued.)

(Continued from page 172.)

Laws V and VI.-The proofs, not astronomical, of these laws, depend upon the apparatus of Law IV., to be seen in the next point.

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No 3.-Applications.-When there is anyt interesting of a historical nature connected w the matter, that too is given briefly under the head of applications.

Application I.-Gravitation, History of.-Although the fact of bodies falling to the earth had been before men from the beginning, there is no record of its having being attended to until the time of Plato and Aristotle. This latter philo.

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Others amongst the ancient philosophers, from time to time, seened to be near what is now held as established.

But the doctrines of Aristotle would appear to have taken most hold of the mind, and ruled supreme with many contained errors, till towards the end of the sixteenth century. Galileo, an Italian philosopher, was the first to show the true principles of gravitation on the earth. Of this more later on. For the present enough to say, that Sir Isaac Newton it was who first declared and proved from the action of gravitation on the earth that all nature might be included in one general law of attraction. A short time ago (1867), the scientific world was startled by a declaration of M. Chasles to the French Academy, that he was in possession of documents which showed that the great Newton's immortal discoveries were due to others, and that, in particular, the fact and laws of universal gravitation were communicated to him by Pascal. Thousands of letters were obtained by M. Chasles, at the price of 140,000 francs, and after a bitter controversy, the Newton Pascal as it was called, of more than two years, he discovered that he was the dupe of a forger. About 22,000 "pieces" of most varied character were produced. A small number, about 500 francs' worth, were genuine.

Application II.-Prince Rupert's Drops.These curious solids, quickly cooled drops of molten glass, Fig. 24, require more than one branch of science for their full explanation, if such has yet been reached. For the present purpose they exhibit a strange mixture of cohesion and non-cohesion. The outside shell is hard and coherent; inside is supposed to be in a state of aggregated fine particles. The cohesion, too, of the shell is exceptional, for, the slightest bit broken off the tail the whole drop is violently reduced to loose dust.

Application III. - Adhesion plates. These sometimes called the " Magdeburg planes," are two discs of metal ground perfectly plane and The copyright of this series of articles is reserved by the Author.

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To prevent side motion, there are three small buttons (d) that can be turned up; or there may be a flange fixed to one of the planes, which however might interfere with their being well rabbed together, and would be in the way of regrinding. The planes are sometimes made of marble or glass, form as in Fig. 26. That their adhesion is not due to air pressure, is shown elsewhere.

App. IV. Smooth bodies.-If left for a long time in contact, smooth bodies with parallel surfaces adhere so forcibly, that it is sometimes not possible to separate them. Thus, Clement found that two pieces of plate glass which had been laid together for a long time, had acquired adhesion such that separation was impossible.

App. V. Uneven bodies.-A thin layer of liquid, water for instance, between two plates & common window glass, will cause them to adhere. The liquid helps to fill up the hollows caused by roughness, and superadds its own adhesion, Fig. 27. This fact is frequently to be met with when

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FIC. 29

solid bodies, wet, are in contact-raise one up straight, others adhere. The upper glass (27) will lift the lower one.

App. VI. Solder, glue, paste, &c-There is produced, by means of these, in a liquid state, an adhesion between solids so close, that it almost

may be called cohesion. That it is not the glue, the paste, &c., which acts in the holding, is proved by this fact: the more they are pressed out the better the joint. Fig. 28, two boards squeezed in cramps, glue forced out.

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App. VII. Donny's figures.-The changing form of liquids adhering to solids has been clearly examined by Donny. A disc is held perfectly horizontal over the surface of the liquid, by means of levelling screws, and the tray is raised or lowered by a screw beneath. All side motion is avoided, and as the tray is gently lower d, the ever-changing beau iful form of the liquid can be steadily observed, Fig. 29. The apparatus serves also for examining the adhesion (and co

hesion) of different liquids. The supports, tray,
&c., are, not shown, to avoid confusion in the
figure.
App. VIII.
Cavendish apparatus.-In the
mountain experiment, the attraction of the earth
for the weight, was a great obstacle to observing
the whole effect of the mountain, The first care,
then, was to relieve the bodies to be experimented
upon from that attraction; next, all external causes
of error to be removed, for clarity points,

1. Two great weights of lead, each 250lb. or more, Fig. 30, were suspended from the ends of a horizontal bar A held through angle rods by a perpendicular axis B.

2. Two very small balls hung from the ends of a light rod of wood d delicately suspended at the middle by a thread of silver, from the top of a box C C C, which protects the balls from the action of the air. Slits are cut in the ends of the box that the lever points a a may be seen. The figure shows the box in section.

a sphere (more or less) that force acts along an
infinite number of radii, Fig. 31, G a, Ga, &c.
App. XI.
"Perpendicular."-A body is said
to be perpendicular when its axis, or some given
side is along one of the radio to the earth's cen-
tre. T tower, radius G B.

WE

(To be continued.)

ANCIENT COINS.-II.

By HENRY W. HENFREY. (Continued from page 169.) E now proceed to describe the series of Roman coins-perhaps the most important of any coinage, either ancient or modern. It is generally divided into two great classes. The first includes all the coins struck under the 3. The box was placed on two massive supRoman Republic and before the accession of ports and levelled by screws SS, in a room where Augustus, which are generally denominated Conno danger of vibration was to be feared. A rod sular or Family coins. "Among antiquaries, d passed through the wall from the top from the those are designated Consular coins which were box, where an endless screw commanded the fix-struck during the Republic with the authority of ing of the lever with its small balls. From out side, therefore, by a nut N, the ends a a could be brought opposite the slits in the end of the

box.

4. To the ceiling of the room, straight over the thread of the small balls lever, was fixed a socket to hold the axis B of the bar A. A pulley pou B supplied a means of turning the axis, and with it the great bails. This was done from outside by a cord F passed round the pulley.

5. The room was without windows, the light of a small lamp L was thrown from outside on the slits, and the points of the lever a a, observed with the telescope T, passed through the wall. All these precautions were to avoid change of temperature or currents of air; no one entered the

room for some time.

6. All being adjusted, the great balls were set at right angles to the small balls, and these brought to perfect rest opposite the slits, which held a divided scale of ivory, the vernier of which was on the ends of the small lever. A ground plan below shows how the great balls could move to the right angle; the dotted lines show their extreme range.

7. The small lever being all but floating in the air, the least ottraction would be observed. All preparations concluded, Cavendish at length moved the great balls near to the small balls; these latter were distinctly attracted, and swung back and forward across a position of rest. There is then the proof of Law IV.

in antiquity to the brass. Silver was not coined in Rome itself until about 207 years B.C., though there are pieces supposed to have been coined in Campania, under Roman authority, as early as 300 B.C. The first that were coined in Rome have Obverse, a laureated head of Janus; reverse, a quadriga, or chariot with four horses. Inscription, ROMA sunken or in relief. Others have, obv., bust of Jupiter; rer., Victory standing near a trophy. The next in date bear, obr., helmeted head of Rome (sometimes called Minerva), X. behind it; rev., Castor and Pollux on horseback, or a chariot with two or four horses.

ten asses.

The principal Roman silver coin was the denarius; so called from its being at first equal to denarii is from 68 to 60 graius troy, and thereThe average weight of the consular fore they would be worth in our money about 8d. or 9d. each. They generally have the numeral of value, X., on the obverse. The other denominations in silver are the Quinarius and the Sestertius. The Quinarius, or piece of five asses, was the half of a denarius, and, when the type of Victory was represented on the obverse, was called a Victoriatus. The Sestertius was the quarter of a denarius, and was sometimes Double denarii or small medallions in silver are marked IIS, behind the bust on the observe. unfrequently found.

The types of the silver consular coins are the most interesting and the most varied of almost any series. The usual obverse type is the head of some deity, but the reverses are extremely persons striking the coins, whether consuls or numerous. The inscriptions include the names of otherwise.

the Consuls, though probably even then under the
superintendence of the triumviri monetales-not
officers in charge of the Mint; those with the
name of any Roman family inscribed, the greater
number of which were struck about the times of
Julius Caesar and Augustus, are included under
the second denomination, but, on account of the
high stations held by many of the strikers, the
two classes are often grouped together as Denarii
Consulares. On these the boundless variety of
type affords an insight into the incalculable
fecundity of the Roman mint, for they present, in
a durable and unequivocal form, names and
attributes both human and divine, sacred rites
and implements, public monuments and edifices,

manners and customs, honours and successes.

They, therefore, form so sterling a record of in-
teresting facts as to constitute, together with
lapidary inscriptions, an incontrovertible evidence
respecting ancient occurrences; and they, more-
over, become a test of validity in the fuller
developement of history, especially in that hitherto
dence of the Republic and the rise of the Empire."
neglected but momentous department, the deca-
-Admiral Smyth.)

The second division comprises all the coins
perial, and usually bearing on the obverse the
truck under the Roman emperors, called Im-
bust and titles of the reigning monarch.

and brass, and the earliest are probably not older
The most ancient Consular coins were of copper
than the third century B.C. They consisted of the
As," and its multiples and divisions. The as
divided into twelve unciæ or ounces. It was, how-
was originally of a pound weight, and was

Cæsar it only equalled balf an ounce.
one or two ounces, and in the time of Julius

For the present it must be enough to say that Laws V.and VI., could be proved by this apparatus and that it served Cavendish and others to calculate the weight of the earth. Baley, in 1842, made many changes in the small balle, and their arrangement, and gave in his results after 3000 experiments. The weight of the earth being deter-ever, as a coin, rapidly reduced; mostly weighing mined, astronomy supplied the means of calculating the mass of the sun, the planets, &c. The Cavendish apparatus has been, then, truly called a balonce to weigh the univesre, This wonderful apparatus was first devised by Mitchel, of the Royal Society, London. He made it over to Wollaston, who gave it, as a present, to Cavendish. This philosopher was the first who brought it to its work. It is, therefore, named after him.

App. IX. Formulæ deductions.-As here are given the first formule, it is well to call attention to their use. Therefore, any one of the four quantities can be known by having the value of the three others. Thus, what would be the attraction of the moon and earth if the distance were to be diminished by half? Here the unknown quantity of the formulæ is G', resolving the equation for G', its value is found. Again, what is the cause of the difference of tides, high and low, ascribed to the attraction of the moon? The formula indicates that it is the greater or less distance of the moon. What is the difference of the action of sun and moon on the water on the earth? The formula of Law VI. will give the relative attraction of the sun and moon, knowing the mass of both, and the formula of Law V. will give the diminution of the sun's attraction caused by its greater distance from the earth.

App. X. Fall of bodies.-They fall because attracted to the earth. "Terrestrial gravitation," ""at length." later on. For the present, it is a force by which bodies are drawn in a straight line to the centre of the earth. The earth being

were:-The Decussis, or ten asses; the Quin-
The various copper and brass denominations
cussis, or five asses; the Quadrussis, or four
asses; the Tripondius, or three asses; the Dupon-
dius, or two asses; the 4s, or piece of twelve
unciae; the Semis, or half-as; the Triens, or
of the as; the Quadrans, or quarter of the as;
the Sextans, or one-sixth of the as; the Uncia, or
one-twelfth of the as; aud the Quincunx, or
piece of five unciæ.

The types of these coins are as follows:-
Decussis.-Obverse, bust of Pallas or Minerva;
X. behind it. Reverse, the prow of a vessel; X.
above it.

Quincussis. Obv., a trident. Rev., a cadu-
ceus.

Various historical events are commemorated, as for instance, a coin of Brutus has a cap of liberty between two daggers, with the inscription EID. MAR. (Eiaus Martie, the Ides of representation of the famous bridge, Pons SubMarch). A denarius of the Emilia family has a licius, built by Ancus Marcius, and afterwards called Emilius. Another exhibits the dream of Sylla, when marching from Nola against Rome, B.C. $3, as related by Plutarch; it is represented by a man lying on the ground, to whom appear Victoria Alata, and Diana. The omen was propitious. A third denarius represents the submission of Aretas, king of Arabia, to Marcus Scaurus,

Among the types are numerous heads of events connected with ancestors, as figure of deities, busts, or figures of ancestral personages, Marcus Lepidus as TUTOR REG. is (the guardian of the king) crowning Ptolemy Epiphanes. Other fications of countries or towns (as heads of Histypes are fabulous monsters (as Scylla), personipania, Roma, Alexandria), heads of allegorical symbolical representations personages (Honor et Virtus, Pavor, Pallor), events, portraits of illustrious living personages of contemporary (as Julius Caesar, Sulla, Pompey), military symbols, (as legionary standards), &c.

Julius Cæsar was the first who obtained permission to put his portrait on coins. His usual titles are "Perpetual Dictator," and "Imperator."

Others

The first gold coinage took place, according to tions are the aureus and the semi-aureus, or halfPliny, in the year 207 B.C. The usual denominadifferent sizes. They were struck in Campania, aureus. The earliest gold pieces are of several and have the word Roma on the reverse. bear the bust of Janus. 40 aurei were struck out of a pound of gold. In In the time of Augustus the time of Pompey an aureus weighed about 126 grains troy, and about 125 in that of Julius Cæsar. By a perpetual law the aureus was equivalent to and Family coins are very similar to those of the 25 silver denarii. The types of the gold Consular silver.

(To be continued.)

ERRATUM. -In our first article, page 169, last line but five, for "Barkobab," read "Barkokab."

Quadrussis.-Generally a bull on each side.
Tripondius.-Obv., head of Pallas; III. behind
it; rev., the prow of a vessel; III. above it.
Dupondius.-The same, but with II. instead of ON THE RELATIONS BETWEEN BODY

III.

As. Obv., double-faced head of Janus; rev., the prow of a vessel; I. above it.

Semis.-Obv., bust of Jupiter; S behind it; rev., the prow of a vessel; S above it.

The triens, &c., are of various types, but usually have the prow of a ship on the reverse. They are marked with large dots or pellets to indicate their value. Thus the quincunx has five dots; the triens, four; the quadrans, three; the sextants, two; and the uncia, one dot.

The silver consular or family coins are next

AND MIND.*
LECTURE I.

(Continued from page 178.)

WHAT are we to say in explanation of the movements? Are they meutal, or are they only physical? If they are mental, it is plain that we must much enlarge and modify our conception of mind, and of the seat of mind; if physical it is plain that we must subtract from mind functions that are essential to its full function,

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