The English Mechanic

AND

MIRROR OF SCIENCE AND ART.

FRIDAY, MAY 6, 1870.

WHAT STRIPES THE SUNBEAM.

BY A FELLOW OF THE ROYAL ASTRONOMICAL
SOCIETY.

THER

HERE are, probably, very few modern discoveries which have excited such widely-spread interest as that of what has been called Spectrum Analysis; in fact, it is almost impossible to peruse, even in a desultory way, much of the current Scientific literature of the day without meeting with references, more or less explicit, to it. Inasmuch, however, as the subject has not hitherto been treated formally in our columns, and inasmuch, further, as it appears to us that a familiar exposition of it may enable many of our readers to appreciate and understand the references to which we have alluded, and which, presumably, have now little or no meaning for them; we have determined to endeavour to place the whole matter in as popular a form as it is susceptible of explanation in, and to tell the marvellous tale of the detection of the chemical constitution of our great centre of life heat and light, the Sun (and even of that of some of those infinitely distant bodies which spangle the midnight sky), by the aid of a mere triangular stalk of glass and a narrow slit in a metal plate.

We must, however, ask here, as we did on a former occasion, for the sustained attention of the reader. It is not a very easy subject which we bave taken upon ourselves to discuss, and in our attempt to make it apprehensible we shall have to proceed, step by step, with, it may be, more or less iteration, in a predetermined sequence of explanation. We would, then, request the Student, on no account to proceed with any paragraph, after the first, until he has thoroughly mastered the preceding one. If he will try his best to comprehend us, we will promise to use all our endeavours to be comprehensible; and to use only such language, and employ only such illustrations, as shall be within the grasp of any moderately-educated artificer who may be sufficiently interested in the subject to follow us.

prism, therefore, being placed in this posture, I they are blended, so that the disc affects the ey e
let the refracted light fall perpendicularly upon a as an uniformly white, or slightly greyish-white
sheet of white paper at the opposite wall of the one.
Brewster ("Optics," pp. 71, et. seq.) pro-
chamber, and observed the figure and dimensions pounded the hypothesis that there were three dis-
of the solar image formed on the paper by that tinct coloured spectra-one red, the next yellow,
light." He then goes on to describe the size and and the third blue-which overlapped, the maxima
shape of this image of the Sun, and to prove that of the red and blue spectra being at the ends, and
and concludes his 2nd Theorem thus:-"This spectrum, and the overlapping producing the in-
the different parts of it are differently refracted, that of the yellow in the middle of the visible
at its least refracted end T, and violet at its most fallacy of this hypothesis has, however, been since
image or spectrum P T, was coloured, being red termediate shades of orange, green, &c., &c. The
refracted end P, and yellow, green, and blue in conclusively shown by Helmholtz, who has proved
first proposition, that lights which differ in colour the superposition of blue and yellow, and that it
the intermediate spaces, which agrees with the that the green, for example, is not made up by
do also differ in refrangibility. The length of the is undecomposable, and of one definite refrangi-
from the faintest and outmost red at one end to of each specific degree of refrangibility is mono-
image in the foregoing experiments I measured bility. The importance of the discovery that light
the faintest and outmost blue at the other end, chromatic, and that each ray has its own special
excepting only a little penumbra, whose breadth colour, will be seen as we go on.
scarce exceeded a quarter of an inch, as was said
above."

So far as we have gone, then, we have arrived
at this result, that what we call white light is in
The experiment whose nature we have thus in-reality a mixture of violet, indigo, blue, green,
dicated is at once so important and so striking yellow, orange, and red lights, and that, after
when seen for the first time, that we would urge falling upon one face of a prism, a beam of Sun-
every one of our readers to repeat it for himself. light emerges from the adjacent face, spread out
lustres "not those cut into many facets, but the violet being the most bent, the indigo less,
The triangular prisms dependent from candle into a sheaf or fan of these its component colours,
the plain triangular stalks of glass-will answer the blue less still, and so on to the red, which
excellently to exhibit the phenomena of which we is bent the least of all. We have further learned
of great use in illustrating our future explanations, that is to say, it is of a fixed and unalterable re-
have been speaking; and as such a prism will be that each of these is legitimately a primary colour;
we would advise everyone who intends to follow frangibility in the same medium, and it is mono-
us through this essay to provide himself with one chromatic, or whole-coloured, and, consequently,
pence each.
or more of them. They may be had for a few undecomposable into any simpler elements. Should
the reader, however, follow our advice, and repeat
Newton's experiment for himself, he will observe
that the colours are not sharply defined at their
junction with the adjacent ones, but that they
spectrum is, in fact, a continuous one, without break
melt, as it were, into each other, and that the
or interruption. It will readily be seen that this
is referable, inter alia, to the shape of the orifice
through which the beam of sunlight is admitted,
as it is pretty evident that the rays which come
through are inclined to each other at the angle
(about half a degree) subtended by the Sun's dise
to the eye. How the use of a round hole in his
shutter caused Newton just to miss a discovery of
proceed to exhibit.
the most vital importance we shall now very soon

It was in the year 1675 that one of the greatest, if not the very greatest, of Englishmen who ever lived, Sir Isaac Newton, made the remarkable discovery that white light was not homogeneous; that is to say, that it could be split, or decomposed, into lights of various colours, and this he ascertained from the circumstance that when, under certain conditions, a ray of white light is refracted or bent, its components are bent in different proportions, or some more than others. He demonstrates this in Prop. 1., Theor. 1., of his "Opticks," published in the year just mentioned, under the heading, "Lights which differ in colour, differ also in degrees of refrangibility." Into his proof of this we need not enter, since we are now more immediately concerned with his second proposition, "The light of the Sun consists of rays differently refrangible." The annexed figure(Fig.1) We may add to what we have said, that, by recopied from the "Opticks," will serve to exhibit ceiving the spectrum as described above, upon a the manner in which he shows the truth of this screen, and then cutting a slit in that screen so as experimentally. Through a round hole F, of in an isolated state, Newton found that such to let light all of one colour, say green, pass through ard of an inch in diameter, Newton allowed the colour was no further decomposable, and that, on Sun to shine upon one face B C, of a triangular letting this ray fall upon a second prism, he only stalk of glass (which we call a prism)," whereby," got green light upon another screen placed behind as he says, "the beam of the Sun's light which the first to receive it. He further showed that by came in at that hole might be refracted upwards examining the spectrum P T through a second towards the opposite wall of the chamber, and there form a coloured image of the Sun. The prism placed below the level of the first, so as to axis of the prism (that is the line passing through reverse the order of refraction, he re-composed his the middle of the prism from one end of it to the other end, parallel to the edge of the refracting the eye as white light, is too recondite a question How or why this combination of colours affects angle) was in this and the following experiments to be discussed here, even were there any tenable perpendicular to the incident rays." This means theory to account for it. It is mainly connected this stage, to glance at the physical properties of Before doing so, however, it will be as well, at nothing more than that a sharp edge of the prism with the anatomy and physiology of the eye itself, the variously-coloured constituents of the specwas downwards, and that such edge was parallel and it is therefore impossible, in this place, to do trum, remembering that all light is due to the to the shutter in which the hole was made, and more than refer to it. We may just say, though, undulations of a medium pervading the whole of to the floor of the room in which the experiment that the phenomenon itself may be exhibited in space, the difference in the rapidity of the vibrawas tried. Having found the position of minimum another form by means of a rapidly-rotating disc tions of this medium affecting the eye as differdeviation of the prism (ENGLISH MECHANIC, upon which the prismatic colours are painted in ences of colour. Numerical details on this subject Vol. X., p. 629), he proceeds as follows:-"The thi: due propertin, and by the motion of which may be found in a letter at p. 35 f h p.s.Lt

coloured band into a spot of white light.

volume.

With regard to the words, "extremred" and "extreme violet," with which the table (1. c.) commences and concludes, we may just observe that, beyond the confines of the ordinarily visible spectrum, exist rays of light both more and less refrangible than any of those which we commonly see; but that by certain artifices or contrivances these may be rendered perceptible. Commencing at the red end, we find that the greater part of the heating effects of the Sun's light are referable to these red rays, and delicate experiments have shown that the maximum heating power resides at a point beyond that at which red light, or any light, ceases to be visible to the eye. Sir William Herschel was the first to draw attention to the fact that heating power mainly resided at the red end of the spectrum, announcing his discovery at the very beginning of the present century; while as early as 1777, Scheele published the results of some experiments, which showed that chloride of silver blackened much more rapidly in the violet rays than in any other part of the spectrum. The preponderance of illumi nating power in the yellow rays must have struck every one who ever viewed dispersed light at all. Summing up, then, we may say that the greatest heat resides in that part of the spectrum just outside of the visible red; the most energetic chemical power just within the visible limits of the violet, and the brightest light about the middle of the yellow.

1

ths of

incandescent gas, what sort of light will that emit? spectrum of potassium, what a wonderful contrast The answer to this is a curious one. Every element does it present! Two narrow lines at the red end in a gaseous state, when heated until it becomes of the spectrum, separated by a dark intervalluminous, gives off its own special and peculiar then a huge dark gap, involving the orange, yellow, spectrum; but this, instead of being continuous, green, blue, indigo, and beginning of the as in the case of that emanating from a solid or a violet, and then one solitary band in the violetliquid, is broken, or discontinuous, and consists of serve to indicate that the burning vapour of potasbright bands or lines, with dark spaces between sium emits lights of practically only two degrees them. A simple and very striking way of showing of refrangibility, and those at opposite ends of this is by the aid of a common spirit lamp, into the spectrum. If we regard the spectrum of the flame of which certain salts are introduced to strontium, we find, as we might expect, that the volatilise and burn in the form of vapour. Makers major part of its light is in the red and orangeof fireworks are familiar with the use of strontium where seven bands may be counted without much to produce a red colour, and of barytes to afford a difficulty; then comes a wide black gap, and anon green one; and we have, most of us, tried the ex- we find a solitary stripe of blue. We have given periment of burning a mixture of spirits of wine these three elements as examples, but had we and common salt in a darkened room for the sake extended them to thirty, our story would have of observing the ghastly effect produced on the been the same. Each and every one would be countenances of the spectators by its monochro- found to differ from all the rest, and to possess matic yellow light. Now it is of the utmost im- its own proper system of bright lines, some at one portance to observe here, that all the salts of end of the spectrum, some at the other, some consodium (the base of common salt) tinge a flame sisting of rays of several degrees of refrangibility, yellow; all the salts of barium, green; all the salts but all being composed, without exception, of of strontium, red, and so on and further, that if bright bands with unmistakeable dark gaps between we can, no matter how, heat the vapour of any them. We may just mention, incidentally, that so chemical element sufficiently high to render it lumi- astoundingly and inconceivably delicate is this nous, we shall find that its spectrum is special to itself, and dissimilar to that of any other body, and method of analysis that the all this irrespective of the ordinary condition of such element, be that solid, liquid, or gaseous. But that we are, for obvious reasons, denied the use of But it is time to return to what we were saying chromatic printing, we should have certainly as to the shape of the aperture through which the given figures of the spectra of some of the alkaSunbeam is admitted to fall upon the prism; and lies, as well as of those of carbon, hydrogen, and we may now proceed to see how Wollaston made nitrogen; but under existing circumstances we (without knowing it) a discovery of very high must forego this valuable aid to explanation. importance in connection with the subject which Prior to entering into any detail as to the kinds we are now considering. Instead of employing, of spectra which we obtain when we regard the inlike Newton, a round hole in his window-shutter, candescent vapours of the various bodies to which through which a number of overlapping spectra we have been alluding, it seems essential here find their way, he let the Sun's light pass through that we should make some reference to the method a narrow slit parallel to the refracting edge of in which these spectra may be observed, and we the prism, and upon now examining the spectrum therefore, to render this more intelligible, here he found it no longer continuous, but crossed in (Figs. 2 and 3) give a figure of one of the simplest several places with fixed dark lines, which were forms of spectroscope, the pattern being one of permanent. Wollaston, however, like many other those of that very able and eminent optician, and discoverers, seems to have quite failed to see the well-known amateur observer, Mr. John Brownimportance of what he had found out, and the ing, F. R.A.S., to whom, probably, this instrument matter slumbered until the year 1814, when the owes more than to any one artist living. great German optician, Fraunhöfer, took the mattor up, and, by viewing the spectrum of a very narrow slit through a telescope, succeeded in we believe, nearly 580 of these lines. Tapping wace called "Fraunhofer's lines," even to this day. He further found out that these lines existed in both direct and reflected sunlight; that moonlight and the light of Venus also showed them, and that in the spectrum obtained from each of these sources, the position of the visible dark lines was fixed and invariable. Fraunhofer made another discovery, too, of very great importance, which was that the fixed stars (which it is well known are themselves Suns) gave speetra containing dark lines, but they were not the same dark lines as he had previously detected in the Sun's light, whether directly or indirectly derived from him.

It consists, in effect, of a very dense flint glass prism P, standing in a truly perpendicular position in the centre of the circle C, which is itself supported by the stand S. The circle C is graduated, and upon it is an upright ring E, fixed firmly, another ring M, attached to an index I (reading by means of a vernier V), moves round the circle C on such index. Into this latter or moveable ring a telescope, M T, of the ordinary construction, is screwed. Into the fixed ring is fastened another telescope, with its object glass also directed towards the prism, but, in place of an eye-piece, carrying in its focus an arrangement of "knife-edges," shown in front view of Fig. 3. This consists of two thin plates of metal s s', with their inner edges carefully ground and accurately parallel, and these are made to approach to or recede from one another by means of the screw For the better explanation of our subject, we with a milled head a, the effect of this being to will now dismiss temporarily from our considera- make an exceedingly clean sharp slit of any width tion the spectra of the celestial bodies, and confine that may be required; a part of this slit is covered ourselves for the present to the effect of heat on by the reflecting prism p, by means of which the terrestrial ones, in so far as such heat leads light from any source placed in the position of to the evolution of light. Every blacksmith knows the candle C, is reflected through it, together that when an iron rod is put into the fire it first with that passing directly between its jaws; so arrives at a red heat, and as it becomes hotter it that the spectra, side by side, can be seen in the gradually emits light of greater and greater re-field at the same time, and the comparison instifrangibility, until ultimately it ends by giving off tuted between them. It is only needful to say at rays of all degrees of refrangibility, or becomes, this stage that the slit formed by the knife-edges as they say, white hot. If the reader will watch sturned towards the light whose spectrum is to be the gradual heating of a common poker by the examined; that of course the rays pass out of the aid of one of the prisms which we have recom- object glass F parallel, fall upon one face of the mended him to procure, he will find that it will prism P, are refracted and dispersed by it, and begin by the emission of red rays; that as the that then they enter the object-glass M, and the heat increases, its spectrum will lengthen; and spectrum is observed through the telescope TM, that orange, yellow, green, blue, indigo, and violet to which the object-glass belongs. light will in succession become visible, the whole Let us conceive, then, that, this instrument of the prismatic colours becoming apparent when being in adjustment, we turn the slit towards the poker is intensely white hot. The same a burning spirit-lamp, into the flame of which we sequence of phenomena will be exhibited by any will commence by imagining that sodium, siy in body, solid or liquid, which can be sufficiently the form of common salt, has been introduced; heated; and it is most important to remark, that and let us proceed to see what sort of a spectrum in each and every case (with the solitary and un- this yields. Certainly a most remarkable one; important exception of an extremely rare sub- a brilliant yellow band, and that is all. The light stance called Erbia which we may quite neglect emanating from this element in its vaporous form here) the spectra of all incandescent solids and is wholly of one degree of refrangibility. Not a liquids whatever, are continuous-that is to say; vestige of green or orange, and, à fortiori, not of uninterrupted and uncrossed by dark lines of blue, violet, nor red-a single bar limited in its any sort or description. But suppose that, instead width by that of the slit, and we have the entire of a solid or liquid body, we are dealing with an spectrum of sodium. If now we examine the

i

180000000 grain of soda has sufficed to show the sodium spectrum, and to flash up in the instrument as a brilliant yellow streak!!!; and further, although it is of little importance to our present object, that this line of sodium, when examined with adequate, optical aid (of which more by-and-bye), is seen really to consist of two lines, like hairs, close together. We may perhaps also add, that we may have as many substances in a vaporous form as we please at the same time in one flime, and that as they successively become volatilised, their spectra will appear one after another when that flame is examined by the aid of the spectroscope, ultimately remaining simultaneously visible until the dissipation of the respective substances which produce them.

(To be continued.)

ELECTRICITY-ITS THEORY, SOURCES
AND APPLICATION.

BY J. T. SPRAGUE.*
(Continued from page 75.)

the op first der oft, is the combination o HE simplest form of galvanic generator, and alternate plates of copper and zinc; most electrical works employ a good deal of space, and many figures, in describing the various forms devised by way of improvement, by Cruikshank, Wollaston, and others, consisting chiefly of the mode of arrangement in the containing vessels, the use of double copper-plates surrounding the zine, and such-like matters, the value of which has been entirely destroyed by further progress. But as the simple copper-zinc arrangement is the most unsatisfactory form known, it is simply wasting time and space to describe such modifications of it, and therefore I shall only refer to it when required for elucidating principles, and as the starting point of all the forms.

109. EXCITING SOLUTIONS.-A current may be generated by two metals in many solutions, but for the single liquid cell the only practical re-agent is sulphuric acid. This is the most powerful, and the cheapest, as it may be bought as commercial oil of vitriol, from about five farthings a pound, and still lower in large quantities. I may as well say here that for the convenience and information of readers I shall thus name the price of all the articles employed; those prices will be those I pay myself, knowing where to go, and are ordinary fair retail, not wholesale or trade prices; at the same time, ordinary purchasers in small quantities, and particularly buyers at chemists, as the retail dealers in drugs are called,

will be asked much more.

Ordinary rectified oil of vitriol of full strength, specific gravity about 1840, will serve, but ordinary brown vitriol should be avoided for two injurious to the silver in a Smee's cell, and cause reasons; if it contains nitrous acid it would be local action on the zine, in spite of amalgamation, in all; to test for this, mix a few drops with water in a test tube, and drop in a crystal of pure

ferrous sulphate, or green vitriol, af er the aci has cooled; if a brown colouration is produced, reject the acid. Another very common impurity

is arsenic, which would be given off with the hydrogen and form a most deadly poison, if much is breathed, and be unwholesome in any case. Sulphuretted hydrogen passed into the diluted acid will cause a yellow precipitate if there is arsenic present. These are the only impurities of any consequence.

Lead gave a momentary deflection of 39°, which fell in a few minutes to 5.

112. SILVER AND ZINC. The silver in this case was better than rolled metal, as it was deposited on electrotype copper, thus giving a favourable surface; this answers perfectly well. If a thick plate is laid on, it gives of course a higher conducting power, and is cheaper, for even the two coatings will contain less silver than a silver sheet of such thickness as to bear practical treatment. It gave 50° falling to 48, 47, 46, 45, at five minute intervals, and 43° after one hour. Like copper, the surface is covered with a brown film, due to the condensation of hydrogen.

A good solution is made by mixing 1 part by bulk with 10 of water, which should be soft, as water containing lime is apt to form a deposit on the metal surfaces; if no other is accessible, it should be boiled with a little carbonate of soda and allowed to cool and settle before nse; 100 grains by measure of such a solution will dissolve about 114 grains of commercial zinc, but it is bad economy to nearly saturate the acid, particularly 113. PLATINISED SILVER.-Smee having asif several cells are combined in series, as zinc is sured himself that the nature of the surface was then pretty sure to be deposited on the lower of the greatest importance, and that the hydrogen part of the negative plate, which is thus destroyed is more readily given off from a rough surface for the time, and from which, as the zine is pure, than a smooth one, and also bearing in mind it is a troublesome process to dissolve it again. that platinum has the highest electro motive But allowing for impurity in zine, local action, force of all the metals as opposed to zinc, deand a due proportion, or about one-fifth of free posited this metal as a fine black powder on the acid left, one pint of this solution would dissolve surface of silver, thus attaining both those obabout 1oz. of zinc. We may thus calculate jects, and yet avoiding the expense of platinum, the work any cell is capable of doing as about and also the low conductivity of this metal, and equal for each pint of solution to 24 equivalents the battery thus arranged, and which justly bears of chemical action, this being the true electrical his name, is one of the most valuable gifts ever unit of "quantity," as I shall hereafter explain. made to electrical science. It is of simple conThe cost of generation for each unit would thus struction, the silver sheet is held in a wooden be that of one pint of solution, 13oz. of zine, frame in a saw-cut down the middle of the inside and a fair per centage for wear of negative plate surfaces; the top and bottom bars may be in. and vessels, divided by 24. This is the acid, and thick, and the sides, the wood being these the conditions applicable to all the follow-well baked and soaked in melted paraffin before ing forms of simple cell. putting together by the usual mortises and tenons, and afterwards weil covered with shellae varnish; a sheet of zinc is held on each face by means of a brass clamp with a screw, which presses them against the frame, and carries also the binding screw for the connection, that for the silver passing through a hole in the top bar, and being soldered to the silver; the zincs should be narrower than the silver, in order to give free escape for the gas.

110. COPPER AND ZINC.-When first a pair of plates are immersed in dilute sulphuric acid, and the wires connected to a galvanometer, a considerable deflection is produced, marking a powerful current, but even in a few minutes the effect rapidly decreases. The following experiment was made for purposes of comparison, with plates 2in. bylin., half an inch apart, in the above-described solution, the plates perfectly fresh, the copper having been washed in nitric acid, and then in several waters; the deflections are those of a Tangent Galvanometer, and therefore capable of comparison, and, as will be hereafter explained, of actual valuation. First deflection, 40°, after successive intervals of five minutes, 34, 33, 32, 31, 31, 30, after one hour, 28, and after an hour and half, 26. The galvanometer and connections consist of about 15ft. of No 13 copper wire, a resistance by actual measurement of 031, or 1-32nd of a British unit.

After a short while the copper is seen to be covered by a dark film, which it is commonly stated is oxide of copper, and the other metals contained in it. This is erroneous however, for it is evident that oxides could not possibly form in the presence of nascent hydrogen; it is really a combination of this hydrogen with the metal, and the diminishing power of the cell is due to the formation of this hydride or alloy,which prevents contact of the copper with the liquid, thereby increasing the internal resistance, while the affinity of the hydrogen in this condensed condition for the acid radical resists the polarising power of the zinc, and therefore diminishes the electromotive force of the couple or cell.

Silver ready platinised may be bought, but it will require occasional renewal, and the process is very simple. The silver, being perfectly clean (it is best to wash it with nitric acid), is placed in a cell containing acid solution, and connected to it is a wire with a small piece of zinc attached, this may be merely wrapped in a piece of blotting paper, or a porous cell may be used to keep it out of reach of the platinum. The zinc is now dipped into the acid to a small extent, in. square is enough, so as to produce a very slight action, and then a drop or two of platinic chloride is added to the acid, and immediately it begins to be reduced on the silver; after this is found to be wholly coloured, the action may be increased, and more platinic solution added, and at the last a brisk action should be produced, the object being, first to secure an adhesive deposit, and then a pulverulent one of platinum. The platinum solution is made by dissolving scraps of the metal in aqua regia, or a mixture of nitric and hydrochloric acid, by the aid of heat. Crystals may be obtained by evaporating to dryness and re-dissolving, but for the purpose required this is needless, as the excess of acid is not injurious.

Pure copper, as deposited by the electrotype Treating thus the plate used in the last experiprocess, has a higher electro-motive power, pro-ment, and using with it one zinc, so as to have the bably because of its purity, but also on account of same conditions as in all the other trials, it gave the nature of its surface, which is covered with 71°; at 5 minute intervals, 66, 63, 61, 59, 57, in innumerable fine points, from which the hydrogen is given off more readily than from a smooth surface. Hence if a copper negative plate is to be used, it should have a deposit of copper formed on it. Such a plate gave the following comparative results--First deflection, 50°, and in successive five minutes, 35, 34, 34, in one hour, 33, and one and a half, 32.

It will be noticed that the greatest loss of power is in the first few minutes; indeed, the first deflection is of scarcely any value for the fall is so rapid that it is nearly impossible to say what the true deflection is, or when the needle has reached its position of rest after the first oscillations.

an hour, 50, and in an hour and half, 46. Like all
the others its power suffers a rapid decrease, in
this case due, chiefly, to the remarkable power
possessed by spongy or pulverulent platinum, of
condensing gases to many times its own volume,
and retaining them in this condition. However,
these comparative tests, though valuable and in-
structive, only treat one aspect of the question of
value, that
of constancy, and relative activity on
short circuit; before coming to a conclusion,
the action has also to be considered, when varying
resistances, such as are met in practice, have to
be overcome.

114. I have long been in the habit of using a 111. IRON AND ZINC.-Iron has often been peculiar plate in my Smee's, one composed of recommended as a positive element because its copper wire gauze, 40 meshes to the inch, which surface keeps clean; its value will be evident I first copper by electrotype, and then silver and from the following results, 25 falling to 18 and platanise; the reason I used it at first was to en16 in ten minutes. Still its cheapness may temptable the zinc to be placed in the middle, and to some to use it, a very unwise economy. For this reason it should be stated that wrought or rolled iron is alone fit for use, as the presence of carbon in cast iron sets up local actions. One reason why iron is unsuitable is its low conducting power, which adds to the internal resistance.

give a freer escape for the gas, and although
the surface is at first view diminished by the
openings, it is not so really, as the whole of the
surface of the wires is active, the back as much
as the front. Such a plate prepared for compari
son gave 69°, falling to 50, 47, 45, 44, and in an

hour and half, 42; this is less than the solid plate, but the reason is that it was not plated sufficiently thickly, and consequently its surface was smaller than it should have been.

115. CARBON AND ZINC.-Mr. Walker suggested the use of graphite plates, and has used them in batteries for telegraphic purposes; this combination gave 25°, falling in an hour to 23, and in an hour and a half to 21. It thus appears that the force is lower than with silver, or even copper, probably on account of the lower conducting power of carbon, but, on the other hand, it has the merit of comparative constancy.

It has also been platinised in the same manner as silver, which greatly increases its powers. As an illustration of the great resistance produced by the hydrogen, I arranged the plate, during platinisation, in connection with the galvanometer, showing a current of 6; the moment the platinum salt was added, and the hydrogen employed in reducing it, the deflection rose to 34°, falling again to 12 as gas began to be again given off. When arranged as in the other cases, it gave 68°, falling to 63, 59, 55, 51, 49, to 37 in one hour, and 31 in an hour and half.

SALINE SOLUTIONS.-As it is well to know what attempts fail, as well as what is successful, I may say that many saline solutions will generate currents, but none are of any use in single fluid cells. Thus, carbon and zinc in common salt, chloride of ammonium, nitrate of soda, and various others gave a deflection of6°, rapidly falling below the power of the Tangent Galvanometer; in fact, the zinc is not able to displace the alkaline or earthy metals which are chemically employed to precipitate it. These solutions have important uses in double fluid batteries, but then other affinities come into play, and these salts act chiefly as conductors, and act on the zine only by being themselves acted on by the other agents, as well as the zinc.

117. Having examined the principal elements employed in batteries which set the hydrogen free, their construction is a matter which need not occupy much attention, for the plan described for the Smee is applicable to all, but without requiring a complete frame; the old fashion of cementing plates into troughs is entirely abandoned, because it is impossible to el an them, a process constantly required, as the copper, when used, needs to be frequently heated to redness to renew its surface, and the zincs ought to be well brushed and freshly touched with mercury after 24 hours' full action, or they lose power, and become subject to excessive local action; but if this care be taken the local action is very small, and it is hardly necessary to remove the plates when not in use, unless a long interval is likely to elapse before they are to be used again, as they keep in much better order in the acid. experiments have resulted in a loss of from a quarter to half a grain per square inch of surface in 24 hours, in solutions of fall strength.

Various

118. Before passing, however, to the compound forms, I will make a present to the readers of the ENGLISH MECHANIC of a cell of iny own arrangement, based on the long series of experiments I have made, and of which the foregoing are a selection. I should have deferred its description till after the examination of the general principles, but so many inquiries have been made lately on this subject that I expect the description will be welcome.

The containing vessel is an ordinary stone jar about 5in. wide by 9in. deep; a strip of gauze 15in. by 5in. is treated as before described, but a copper plate silvered, or sheet silver would do, though I believe not so well; this is made into a cylinder fitting inside the jar, and slung by three copper hands or wires (soldered to it and covered with cement or gutta percha after completion) so as to leave a space of 3in. in the jar below it; the suspending wires are turned over to the outside and soldered to a band going round the jar and fitted with a binding screw, a wooden cover is fitted with a hole in the middle, through which passes the connection to the zinc. The zinc is a strip 2in. wide by fin. thick, and at its foot is secured a little shoe or saucer of gutta percha containing mercury, which maintains the amalga mation. Of course the negative element is somewhat expensive at first start, but the object is to give it as large surface as possible; a series of carbon plates might be substituted, but they would cost as much as the silver and have no residuary value, and give less force; the platinising is done at the first charging, taking care to reduce all the platinum before inserting the proper zine.

This battery is nearly constant, it deflected my