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"WHAT STRIPES TUE SUNBEAM.

вт A. Fxllow Of The Royal Astkokomical Society.

( Continved from page 173.) T^VEHYBODY who has scaled a letter with a -ej lighted taper in a room into which the Sao was shining, must hare noticed that the candle-flame cast a perfect shadow; so that »ach shadow, arter all, is only a question of the relative iutcn«ity of two lights. We may hence see how it is possible that, althongh this line D may Ьате some luminosity, yet, stopping ont 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 Roecoe, in one of the splendid lectures which he delivered in May, 18C8, 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 beck, and looked absolutely dark and smoky when optically superposed on it. He subsequently veponrised other salts in the small front flame ; hut as the rays emitted were of a different refrangibility to these issuing from the great flame, they no loDger absorbed any of its light, nnd the smoky appearance entirely vanished.

If we thoronghly 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 refraogibility as those which they emit; that, therefore, their spectra can be reversed—that is to say, that when Jight of fufficient brilliancy giving a continuous spectrnm 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 spectrnm 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 the.-e 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 ri« «tí«, a resemblance, or rather identity, extending to the minutest particulars.

It would be worse than childish to regard this « mere chance. Two lines might ngree fortuitoutly j 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, ihe light which forms the Solar spectrum mutt have patted 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 veryeasy to understand how our own atmosphere eon Id contain a sufficient amount of iron vapour tj yield such exceedingly sharp and distinct ab

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sorption lines ss are visible in the Solar spectrnm; 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-increased thickness of air. Ou the other hand, the very high temperature which it is only reasonable to assume most 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 certainty as we can attain in any question of natural science."

He goes on to add: "As soon as the presence of one terrestrial element in the solar atmosphere was thus determined, and thereby the existence of a large number of Fraunhofer'» lines explained, it seemed reasonable to suppose that other terrestrial bodies occur there, and that by exerting their absorptive power they may cause the production of other Fraunhofer line». For it is very probable that elementary bodies which occur in large quantities on the earth, and are likewise distinguished by special bright lines on their spectra, will, like iron, be visible in the sol«r atmosphere. This is found to bo the case with calcium, magnesium, and sodium. The number of the bright lines in the spectrum of each of these metals is indeed small, but those lines, as well as the dark ones in the solar spectrum with which they coincide, are so uncommonly distinct that the coincidence can be observed with very great accuracy. In addition to this, the circumstance that these lines occur in groups renders the observation of the coincidence of these spectra more exact than is the case of those composed of single lines. The lines produced by. chromium form also a very characteristic group, which, likewise, coincides with a remarkable group of Fraunhofer's lines; hence I believe that I am justified in affirming the existence of chromium in the solar atmosphere. . . . Barium, copper, and zinc appear to be present in the Solar atmosphere, but only in small quantities; the brightest of the lines of these metals correspond to distinct linea in the Solar spectrum, but the weaker lines агз not noticeable. The remaining metals which I have examined—viz., gold, silver, mercury, aluminium, cudmium, tin, lead, antimony, arsenic, strontium, 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.

It must not, however, be supposed that all the dark lines visible in the solar spectrum are referable to vapourised elements in his atmosphere. Some of them, undoubtedly, have their origin in our own. The late Sir David Brawster was the first to point this out. M. Janssen, the famous French physicist, whom we shall have occasion to refer to again further on, experimented on light passed through a column of high-pressure steam, and found that the steam exerted a stronglyabsorbing power, groups of dark lines appearing between the extreme red and the line I). As the additional lines seen is the Solar spectrum when the Snn 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 vaponr of our own air.

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

explained? Kirchhoff's idea, propnnnded in his book, is that the Sun consists of a solid or liquid nucleus heated to the brightest whiteness, and giving a continuous spectrnm, and surrounded by an atmosphere of somewhat lower temperature, containing the vapours of the various substances, which, by stopping off the rays corresponding iu refrangibility to those of their own bright spectra, filter, as it were, the solar light on its way to ns, во 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 nt Guntoor, in India, on August 18, 1868, and have there very briefly referred to the discovery that the wonderful masses of red flame (which have been at once the greatest admiration of, and pHzzlo to, former observers), visible when the Sun's disc was finally obscured, consist of stupendous flames of incandescent hydrogen gas. Prior to the date of which we speak, tbese astonishing objects had never been perceptible, save under the circumstances of a total eclipse, but, finding that they yielded a bright line spectrum, it struck M. Janssen (of whom we have previously spoken, and who went out to Guntoor as the French Government astronomer) that they or rather their spectra, might be observed at a time when the Sun was unobscured—an observation in which he was successful on the very next day. It happened, very "buriously, that Mr. J. Norman Lockyer, F.R.S., arrived, quite independently, at the same ca viction as M. Janssen, and, without any communication with him, also succeeded in making on, the spectrum of these prominences, Mr. Lockyer who finds that these gigantic uprushes of by-' drogen are only local aggregations of an envelope surrounding the Sun, proposes to call this envelope tbe chmmotphere, to distinguish it from the absorbing atmosphere on the one hand, and from the white-hot photosphere on the other. Into this part of the subject, however, it is not very important that we should enter into much detail. Research is still being sedulously carried on, and the time has scarcely arrived for anything more than a hypothesis which shall ' account generally for observed appearances. We will turn, then, to the results of spectroscopic research as applied to the fixed stars and nebula:.

Before, however, we do so, it may be as well to ravert to the instrument by means of which our observations must be made, and to say a few words аз 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 spectroscope with the practical working of which we are oorselves more familiar, and which is infinitely better suited to the possessors of moderate telescopes than the costly and complicated one we have alluded to. It must suffice, then, to say that Mr. Huggine has two prisms (to increase the dispersion) in his instrument, and that, by a very delicate adjustment, he gets and retains a star between the jaws of the slit of it. The star, being a point, will be lengthened into a mere line of light, so has to be spread out by a cylindrical lens into a band. There iä the usual provision of a diagonal prism over half the slit for the direct comparison of any particular spectrnm with that of a star, planet, or nebula. Premising that a spectroscope is worse ihan 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 ciivider the most suitable for an aim.t-'ur. It iiduutB of no mtasurcinettl ot the spoctra, bnt this, as will bo 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 11,18G9. It is shown in section in Fig. ft, 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 ou the slit C, by means of a sliding tnbe, H; both the prisms and lens are fastened in this tube. K is a small rightangled prism eo»aring 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. D D is a ring milled on the edge ; on turning this round both edges of the slit are made to recede from oach other equally, beiug acted oo by two hollow eccentrics. The lines can thus be increased in breadth without their centres being changed—a point of importance. B is a cylindrical lens attached to tho tube F, which slides into another tube, G. To use the spectroscope, tho adapter L, which is the same size as tbo Society's thread, has only to bo screwed into the eye-drawer of the telescope in the place of the ordinary Huyghonian eyepiece. The drawer must then be adjusted so that the slit G comes exactly to the focus of the object glass. This ahonld be triod beforehand by getting an image of the Sun, and then making a mark on the drawer-tube of the telescope. When this has been done the tube can be set by this mark, and the spectroscope screwed into it at any time without any troublo in adjustment. If the cylindrical lens be removed the spectrum of a star will tin a mere line of light. Tha cylindrical lens is for the purpose of widening this line to such an extent that the lines in tho spectrum may readily be discerned. For this purpose tho Ions must be placed with its axis at right angles with Ike slit, and the best distance from the slit is between three and six inches. The nearer it is brought to the slit the broader will be the spectrum; bnt ir. should not be used too close, an account of the diminution of the light. When it is desired to •litfiin tho spectra of planets, comets, and nebula), or, indeed, any heavenly bodies possessing considerable diameter in the telescope, the cylindrical lens may be dispensed with advantageously."

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

(To be continued.)

COLOUR AND COLOUR BLINDNESS
By "omickon."
(Concluded from page 170.J

"OROFKSSOR WARTMANN notices that there -*■ is prevalent an opinion, that the number of Daltonians who have blue eyes, is greater than the number possessing black eves, but this opinion he confidently denies, and M. Scebeck entirely coincides with him. In the number of Daltonians examined by him, the majority bad black eyes, and in reckoning up all tha cases of colour-blindness in which the tint of the iris has been noticed, ho finds that the two colours are in equal numbers. Prof. Wartmann neticed, howover, without laying any great weight on the 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."*

Dalton accounted for tho peculiarities of his eye on what may bo termed the "Chromatic Theory." He noticed "that a sky-blue transparent liquid modified the light of a candle, so as to make it similar to daylight, and of course restored to pink its proper colour by day—namely, light blne."t This induced him to publish his theory tbus:—"it appears,thcrefore,aluiost beyond

"Taylor, " Scientific Memoirs," lslfl. t "Manchester Memoirs,' 17'JS.

u doubt, Unit oiiu ol tUu Uimiouia of my eye, and of tho eyes of my follows is a co'oured medium, probably so:ni modification of blue. I suppose it must be the vitraons humour, otherwise, I apprehend, it might be discovered by inspection, which has not been done."# As Sir David Brewster has said in a paper on " Co'our Blindness," that appeared in the " Philosophical Magazine," of 1844, this hypothesis of Dr. Dalton is the onlyone capable of boing proved or disproved. Let us see to what result the attempt to prove it has led.

Mr. Joseph A. Ransome examined Dalton's eyes after he wos 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 crystallino lens was slightly amber-coloured, as usual in persons of advanced age. The tunics retina, choroid, and sclerotic, with their subdivisions, presented no peculiarity, f And further, after the posterior portion of the outer coals 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 daring his life did not gain many supporters, and after his (loath 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 tho contrary, a very strong argument. Perhaps this argument is not of any groat 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.

Sir David Brewster, in a paper that appeared in the " Philosophical Magazine," attempted to account for the peculiarity of colour blindness, by a supposition very nearly akin to that of Dr. Dalton, but he rejected the supposition of a coloured vitreous humour, and substituted a coloured retina. We give his conjecture in his own words :—" During the dissection of many hundred eyes, I observed in several cases that the retina had a marked French grey or pale blue tint, which decidedly absorbed red light. I knew that in cases of colour blindness the vitreous humour was not blue, or even greenish blue, as Dr. Dalton conjectured, but I could not assert that in the same oases the retina might not be blue, and hence I was led to hazard the idea of a blue retina, as one which might he admissible as a cause of colour blindness, but only on the supposition that the choroid coat should prove to be the seat of vision. Tho italics are Sir David Brewstor's. I do not think that Sir David Brewster ever attempted to extend this supposition 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.

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

The Rev. Philip Kelland has published a theory or suggestion to account for Chroma Pseudopsis, which deserves to be mentioned. He regards the sensation of colour as due to tho reaeption 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 may be presumed to vibrate agreeably to their period.

In the normal state, the retina is supposed to be capable of responding to pulsations of every period within cortain limits, and of conveying to the mind the sensations which we term colour appropriate to each period, but in other states of the retina this appears incapable of being effected, but a vibration occurs more readily i:i accordance with one period of pulsation thnn with another.

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i'iiis coi iima-s one form of chroin ue psendopsis. Another form in which the mora strongly marked complementary colours are mistaken, Profc-sor Ke land supposes to arise by the facility pcBjessca by tho rctin* for starting into vibrations of every period, when excited by thosj of one only, "just as a musical pipe or string gives all the notc3 of the chord at the same time, or as the different strings of a harp vibrate when only one has been struck." Professor Wartmann, in a second paper, suggested a theory nearly similar to that of Professor Kclland, in which he says, " I admit that the seat of colourblindness 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 vibration caused by a red ray is identical with that which a green produces, thorc will be confusion of these colours. This theory is independent of every system destined to explain light." Whether the reader may close to adopt either of these theories or not, it seems generally evident that the cerebro-retinal apparatus of vision in tho eoloHr-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.

If any of our readers have discovered that their eyes possess the peculiarity of whioh this paper treats of, they may desire to know whether their case admits of cure or palliation. We regret not to be able to offer any very great hope respecting an ultimate and complete cure, when tho oolour-blindness has been congenital, but there are a certain class of diseases whioh for a time causo the sufferer to experience all the announce and coi fusion that congenital colourblindness is calculated to produce, and differs from it only in the respect that appropriate treatment may cause it to disappear. In the case of colour-blindness, that was occasioned to a gentleman by a fall from his horse, that we quoted in the earlier part of this article, the peculiarity has irremoveably attached itself, and does not apparently admit of cure. Dr. Erie (himself u member of a family in which seventeen cases of this affection of vision have occurred), holds an opinion which we believe is Lot generally supposed by medical men, or observed fact, that " a succession of years modifies tho Daltonism of the same individual," though he does not hazard an opinion as to what extent advancing years induce a change. Dalton was as colour-blind at the Oxford meeting of the British Association, when he compared the colour of his D.C.L. gown to that of the leaves of the tree, as in 1791', when he first discovered hii colour-blindness, and as he remained to the day of his death in 1844. Moreover, there is not on record one case where a cure of congenital colour-blindness has been effected, thongh nearly all would make some attempt to cure themselves of this defect, and, as a rule, the more observant and the better eJu 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 thera "exists a very easy means of rectifying, to a certain extent, tho 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 tho 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 art tidal 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 tlrut neither impression i< 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 au orange or yellowish appearance, which forms a strong contrast to blue." As a method of palliation, Dr. Wilson suggested employing glass, stained yellow i.r orange, by means of oxide of silver, and iu many cases he mentions these classes proved of •rest service to his colour-blind acquaintance. Soeh experiments *s we ourselves have been able to make on these means, as a palliative to-colour blindness, have not convinced us of any ntilitythat they are to the general colonr-blind public. Onr investigations have led ns to conclude that the media employed, to be of any use, must be selected by experiment by the Bnfferer himself. Let him cboose 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 ns 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 colouredblicd, to endeavour to detect the colour, and not the particular shade or tint, that the colour presents to a normal eye.he*ion) of different liquids. The supports, tray, A.-., are, not shown, to avoid confusion in the figure.

THE END.

THE INSTITUTION OF CIVIL ENGINEERS

WE have j ust received a report of the speeches at the annual dinner of the Institution of Civil Engineers. One would have thonght 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 Bpoeohes of Lord Granville, Lord Derby, and the Archbishop of York. But scarcely any allusion was made in. these reports to the Engineers, or to Scicnee at all—or, in other words, Science, at a meotiug of scientific men, was only incidentally alluded to. Now, who i9 to be thanked or blamed for this? Nobody but the Engineers themselves. One would have thonght that at an annual meeting of one of our principal scientific bodies the chief men connected with that body would have had something to tell tho world worth listening to about the progress of Science. Instead of this wc have men quite able, we admit, to instruct the public on matters legal, social, or political, but altogether inadequate to sav anything worth listening to on scientific matters. We should have thought that the Institution of Civil Engineers 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, adopting snch 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 tho Army and Navy, the Bar, the House of Lords, and the Commons, to respond to toasts in a festive way; bnt 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 Engiueera, is simply ridiculous.

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ELECTRICITY—ITS THEORY, SOURCES, AND APPLICATION* By J. T. Sprague. ( Contin ued from page 148) "I JQ As a suitable appendage to the simple 1J v« battery described on page 147, amode of economising in the article of ziuo deserves attention. Smee proposed what he called mi "odds-and-ends" cell, oomposed 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 hns been tried by many, but for many reasons has never given satisfaction, bnt 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 plnnged a stout copper wire, amalgamated at its end, bnt covered everywhere else with gutta percha, and cemented to the side of the vessel, reaching to its top, where it is to be

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soldered to a binding screw which is the zinc connection. All the rest of the surface is pierced with as many holes as possible consistently with strength, to allow free circulation of liquid. It is then filled up with pieces of zinc, amalgamated and in as close contact as may be; the wholo acts and is nsed just ns if it were an ordinary plate. The mercury is subject to little waste, but now and then tho whole should be removed, well shaken up together, and repacked, and at times the mercury as it becomes charged with metals should bo 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 nsed either in snch a battery as my arrangement, or in a Daniell or other cell, within a porous jar.

120. Two Fluid Cells.—All the combinations described so far have two faults. 1. Weakness —Owing to the force being largely absorbed bv the escaping hydrogen, hence their"electro-motive force is low, and any large resistance greatly reduces the current tbey can yield. 2. Want of Constancy—Ab shown 'by the rapid fall in the experiments, though in my own form this is in great degree overcomo by largely increasing the negative surface. For many purposes, constancy is essential, and it is desirable in all, hence continual efforts have been made to overcome these two defects, aDd with considerable success, though a really constant battery has yet to be discovered, notwithstanding the praises bestowed by manufacturers and patentees on several forms. As yet constancy is only to tie obtained by the use of two agents, one acting on the zinc, the

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other absorbing the hydrogen a* the negative pole, and the success is greater just hi proportion 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 separating medium, as to which a few practical observations will be of valne.

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 pnrposes, 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 othor 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. Tho 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 nee. 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 mert-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 dene the salts rise up, effloresce, 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 thein wherever the zino 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 Dajtiell's Cell.—fbis, 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 keptconstantly renewed. The acid of the salt is transferred by electrolysis to the positive metal, through tho porous medinm, hence if a fresh supply of the salt is added to the solution to replace that removed, this part of the arrangement remains constantly in the same condition; but still absolute constancy cannot be obtained, because as ihe zinc dissolves, the solution belonging to it becomes less active and less conducting. Tn« great drawback to this cell is that the copper salt parses by endosmose into the zinc solution, and acts on the zinc where tho copper is deposited, and causes great waste by setting up local actions.

This defect may be to a great extent remedied bv arranging a piece of wire ganze close to, but not touching the porous diaphragm, on the zinc side, in such a manner that any fluid passing towards tho zinc must go through the gauze; this is connected by a wire to the copper plate during action, it thus becomes electrically part of the negative surface, and in same cases will act as a single fhrid 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 zinc; when the battery is »ot acting the gauze should be conneoted to the zinc by a line wire of iron or brass, so as to keep np a very alight action :or the same purpose.

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SCIENCE FOR THE YOUNG."

By The Rev. K. Kkbnan, Clongowes College.

(Continued from page 172.)

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

No 3.—Applications.—When there is anyt 1 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 pliilo

cylinder within the reservoir is of course per forated. Modes of construction may be varied to any extent. Thusiustead of B 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 -4- 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 the 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 b

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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 aud 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 tort, that is to say fitted with plates in cells, either di fided 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 cnlosmosc.

(To be continued.)

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Others amongst the ancient philosopher*, from time to time, seemed to be near what is now held as established.

But the doctrines of Aristotle) would appear to have taken most hold of the mind, aud 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 trom 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. Cbssles 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, tho 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 button* (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 rubbed 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 lone 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 a: common window glass, will cause them to adhere. The liquid helps to fill up the hollows caused by roughness, and superadds its own adhering, Fig. 27. This fact is frequently to be just with when

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App. VIII. Cavendish apparatus.—In the mountain experiment, the attraction of the earth for tbe 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 olarity points,

1. Two great weights of lead, each 2501b. 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 (he ends of the box that the lever points a a may be seen. The figure shows the box in section.

3. The box was placed on two massive supports and levelled by screws S S, in a room where no danger of vibration was to be feared. A rod d passed through the wall from the top from the box, where an endless screw commanded the fixinit of the lever with its small bolls. From outside, 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 p ou B supplied a means of turning the axis, and with it the great balls. 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/ohserved 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.

G. All being adjusted, the great halls were set at right angles to the small bulls, and these brought to perfect rest opposite the silts, 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 bat floating in the air, the least ottraction would be observed. All preparations concluded, Cavendish at length moved the great balls near to the small bolls; these latter were distinctly attracted, and swung back and forward across a position of rest. There is then the proof of Law IV.

For the present it must be enough to say that Laws V.and VI., couldjbe proved by this apparatus and that it served Cavendish and others t» calculate the weightof the earth. Baley, in 1842, made many changes in the small balls, and their arrangement, and gave in his results after 3000 experiments. The weight of the earth being determined, astronomy supplied the means of calculating the mass of the snn, the planets, &c. The Cavendish apparatus has been, then, truly called a balonce to weigh the univesre, This wonderful apparatus was first devised by Mitohel, of the Royal Society, London. He made it over to Wollaston, who gave it, as a present, to Cavendish. Thia philosopher was the first who brought it to lta work. It is, therefore, named after him.

App. IX. Formula} deductions.—As here are given the first formulrc, 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 formula; 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 tha 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 cent-e of tbe earth. Tl e esrth being

a sphere (more or less) that force acts along an infinite number of radii, Fig. 31, G a, G a, 4c.

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 centre. T tower, radiuB G B.

(To be continued.)

ANCIENT COINS.—II.
By Henry W. Henfrky.

(Continuedfrom page 1C0.)

WE now proceed to describe the series of Roman coins—perhaps the most important of any coinage, cither ancient or modern. It is generally divided into two great classes. The first includes all the coins struck under the Roman Republic and before the accession of Augustus, which are generally denominated Contnlar or Family coins. "Among antiquaries, those are designated Consular coins which were struck during the Republic with the authority of the Consuls, though probably even then under the superintendence of the triumviri monetalcs officers in ohnrge of the Mint; those with the name of any Roman family inscribed, the greater number of which were struck about the times of Julius Oosar and Augustus, are inoluded under the second denomination, but, on account of the high stations held by many of the strikers, the two classes arc often grouped together as Denarii Consularcs. 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 interesting facts as to constitute, together with lapidary inscriptions, an incontrovertible evidence respecting ancient occurrences; and they, moreover, become a test of validity in the fuller developenient of history, especially in that hitherto neglected but momentous department, the decadence of the Republic and the rise of the Empire." —(Admiral Smyth.)

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

Tho most ancient Consular coins were of copper and brass, and the earliest are probably not older than the third century B.C. They consisted of the "As," and its multiples and divisions. Tho as was originally of a pound weight, and was divided into twelve vne're or ounces. It was, however, as a coin, rapidly reduced ; mostly w.ighinir one or two ounces, and in the time of Julius Cresar it only equalled half an ounce.

The various copper and brass denominations were :—The Deonuis, or ten asses ; tho Qnincuts is, or five asses; the Quadrtusis, or four asses ; the Tripondius, or three asses ; the Dupondius, or two asses ; the As, or piece of twelve unci*: ; the Semis, or half-as; the Triens, or J of the as; the Quadrant, or quarter of the as; tho Sextans, or one-sixth of the as ; tho Uncia, or one-twelfth ot the as; aud tho Quincunx, or piece of five uncia:.

The types of these coins arc as follows :— Becussis.Obverse, bust of Pallas or Minerva; X. behind it. Reverse, tho prow of a vessel ; X. above it.

Quincitssis.Obv., a trident. Rev., a caduccus.

Qnadrussii.—Generally a bull on each side.

Tripondius.Obv., head of Pallas ; III. behind it; rev., the prow of a vessel; III. above it.

Dujwndiits.—The same, but with IL instead of

in.

As.Obv., donble-faced hcadofjanns; rcr., the prow of a vessel ; I. above it

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

The triens, &c, are of various types, bnt usunlly have tho prow of a ship on the reverse. They are inarkod with large dot" 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

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

The principal Roman silver coin was the denarius; so called from its being at first equal to ten asses. The average weight of the consular denarii is from 68 to 60 graius troy, and therefore 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 Qu {.nartu* and tho Sestertius. The Quinarlvs, or piece of five asses, was the half of a denarius, aud, when the type of Victory was represented on the obverse, was called a Victoriatus. The Sestertius was tho quarter of a denarins, and was sometimes marked IIS, behind the bust on the observe. Double denarii or small medallions in silver are not unfrcquently found.

The types of the silver consular coins are the most interesting and the most varied of almost any Beries. The usual obverse type is the head of somo deity, but the reverses are extremely numerous. The inscriptions include the names of persons Btriking the coins, whether consuls or otherwise. Various historical events are commemorated, as for instance, a coin of Brutus has a cap of liberty between two daggers, with the inscription En>. MAE. (Eidns Martiat, the Ides of March). A denarius of the -Emilia family has a representation of the famous bridge, Pons Subliciiis, built by Ancus Marcius, and afterwards called JZmilius. Another exhibits the dream of Sylla, when marching from Nolo against Rome, B.C. S3, 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 deities, busts, or figures of ancestral personages, events connected with ancestors, as figure of Marcus Lepidus as Tutor Reg. is (the guardian of the king) crowning Ptolemy Epiphanea. Other tvpea arc fabulous moDSters (us Scylla), personifications of countries or towns (as heads of Hispania, Roma, Alexandria), heods of allegorical personages (Honor ct Virtus, Pavor, Pallor), symbolical representations of contemporary events, portraits of illustrious living personages (as Julius Cocsar, Sulla, Pompcy), military symbols, (as legionary standards), &c.

Julius Ctesar was the first who obtained permission to put his portrait on coins. His usnal titles are " Perpetual Dictator," and " Iinperator." The first gold coinags took place, according to Pliny, in the year 207 B.C. The usual denominations are the aureus and the srmi-aureus, or halfnureus. The earliest gold pieces are of several different sizes. They were struck in Compania, and have the word Roma on the reverse. Others bear the bust of Janus. In the time of Augustus 40 aurei were struck out of a pound of gold. In the time of Pompey an aureus weighed about 126 grains troy, and about 125 in that of Julius Ctesar. By a perpetual law the aureus was equivalent to 2~> silver denarii. The types of the gold Consular and Family coins are very similar to those of tho silver.

(To be continued.)

Erratum.—In our first oiticle, page 160, last lino hut-five, for " Barkobab," read '• Barkokab."

ON THE RELATIONS BETWEEN BODY

AND MIND.*

Lectubb I.

(Continued from page 178.)

WTfAT are we to sny in explanation of tho movements? Are they mental, or nre they only physical? If tboy are mental, it is plain that we mutt 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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