mities with platinum points. They may be caused to approach each other up to contact, and they can be diverted so that a point shall stand opposite a disc, or both discs opposite to each other. In addition to the conducting wires which proceed from the induction coils, several wires for the passage of the current for other purposes may be attached to the discharger.

5. The condenser is known to be a very important part of the apparatus. Prof. Poggendorff and M. Sinsteden have already proved that it may be constructed in different ways. It is important to arrange it so that it may be used to increase or diminish the extent of charged surface. One attempt to construct the condenser in this manner I pass over, because no good result was obtained, at least it did not produce the effect of an ordinary wax-cloth condenser of the same surface. Nevertheless I have not found the fear expressed by M. Sinsteden, that the wax-cloth condenser would be easily broken through by the sparks when the excitement is intense, fulfilled. In the drawer M is an ordinary wax-cloth condenser 8 Par. feet long, built into layers separated from each other by varnished partitions, and connected with the binding screws which stand at the front of the drawer. The attempts to construct a condenser out of single pairs of plates I have not yet given up, as it possesses great advantages. The condenser of M. Sinsteden, formed of plates of glass, fulfilled indeed its object; I believe, however, that on account of the thickness of the glass, it must have a greater surface than the wax-cloth condenser; and besides this, such a condenser is very fragile, as the thinnest glass must be made use of, and the plates, though in a drawer, must be permitted to stand free.

6. Experiments with the apparatus gave a development of induced currents which, to my knowledge, has never been attained hitherto. The change of the sparks passing between the platinum points of the discharger, when the condenser is introduced, is very striking. While without the condenser the sparks are very slender, are but feebly luminous, produce a fine crackling noise, and reach a length of only 5 or 6 lines, they immediately assume a wholly different character when the condenser, one side of which is in connexion with the lever C, and the other with the copper spring J, is introduced. The sparks become more massive: they are at least four times as wide, and surrounded by coloured fringes; the report is loud, particularly when those of greater length and rarer occurrence spring over. When the room is rendered dark, a brush of electricity, like that of the electric machine, is seen streaming from the points of the discharger, the positive being easily distinguished from the negative. By the application of one or two of Bunsen's cells, sparks are ob

tained from 12 to 14 lines in length, which do not follow the direct line joining the points, but sometimes describe considerable zigzag curves. Such long and rarely occurring sparks are, however, only obtained when the lever C is pulled as strongly as possible, so that the pressure of the platinum point on the copper spring is very strong. When the points of the discharger are caused to approach within about 8 lines of each other, a continuous stream of sparks passes, which becomes more intense when the play of the interruptor is permitted to proceed more quickly. The play of sparks at the discharger is very beautiful when a bit of chalk, sugar, glass or wood is placed between the points. Wood-shavings, paper, cotton-wool, and gunpowder, are immediately ignited by the sparks.

As these induced sparks develope a comparatively greater amount of heat than those of the electric machine, even when they proceed from the strongest battery, it may be expected that such instruments will find application in the art of war, more especially as preliminary experiments in France, with far feebler apparatus, have already given satisfactory results. The ignition of gunpowder, for example, is immediately effected when Varrentrap's fuse is made use of. This fuse consists, as is known, of a mixture of 2 parts of chlorate of potash and 1 part of sulphide of antimony, and is spread over the gunpowder. If through this mixture the smallest induction spark is permitted to pass, it is immediately ignited. Six such grenades, which were connected one after another at great distances, I have completely and simultaneously ignited. The insulation for the induced currents is far less difficult than for those of the electric machine; the apparatus itself is very small, not dependent on the state of the atmosphere, &c. Of course, for practical purposes, the instrument would be constructed in a totally different and more simple manner than that just described. The interruptor, for example, would be formed of a single presser, similar to the key of a Morse's telegraph. The induction coil and condenser could be so preserved in a box that injury would be impossible, as neither of them is fragile. This is not the place to follow this purely technical subject further; it results, however, from what has been said, that this electricity is more suitable for the purpose mentioned than the frictional electricity recently applied in Austria.

I partake less of the hope expressed by M. Sinsteden with regard to the possible application of induction currents for igniting the coal-points for technical purposes. My apparatus ignites the points very prettily. The coal is even rendered glowing; but the intermittent character alone of the induced current would deter me from further experiments, for such a light could never

be made applicable for technical purposes. That the light from the points, even when the interruptor moves most quickly, remains very plainly intermittent, may be easily proved by causing an illuminated object to move quickly to and fro. In general the induction current will always develope less heat than the voltaic current applied to produce it; and although this property does not stand in direct relation to the generation of light by the coal-points, still we cannot think of producing the latter without a strong development of heat.

VIII. On some New Methods of producing and fixing Electrical Figures. By W. R. GROVE, Esq., V.P.R.S. &c.*

ACLASS of figures produced on polished surfaces by electrizing

a metallic bas-relief, such as a coin or medal placed on glass, mica, or polished metal, was made known by M. Karsten†, who refers in his memoir to the previous results of Moser and Riess, the latter having given the name of roric figures to those produced by electrical discharges on account of their becoming visible, as Karsten's did, when breathed on. M. Karsten states that he had but imperfectly succeeded in fixing these figures by exposing them to the vapours of iodine or mercury ‡, and that when an insulating substance was interposed between the object and the recipient plate, the figures were not formed §.

This class of experiments possesses much interest as showing the molecular changes accompanying electrical phænomena; and believing, as I have for many years, that electricity is nothing else but motion or change in matter, a force and not a fluid, I have recently made some experiments to ascertain whether similar effects took place in cases where electrical light is visible upon insulated surfaces only, a great number of experiments having already shown that the particles of metals or conducting bodies are projected when the electrical spark proceeds from them.

M. Du Moncel has shown that when two plates of glass coated respectively on their exteriors with metallic plates are kept separate and electrized, a brilliant electrical light is seen between the plates . I thought I might render evident the molecular change which I believed to be taking place on the opposed surfaces of glass in such cases, and the following experiments, selected from many others, will, I think, prove that this is the fact.

1. Two plates of window glass, 3 by 31 inches, were immersed in nitric acid, then washed, and dried by a clean silk-handker

* Communicated by the Author.

† Archives de l'Electricité, vol. ii. p. 647; iii. p. 310; iv.

Ibid. vol. ii. p. 651.

p. 457. § Ibid. vol. iv. P. 464.

Notices sur l'appareil de Ruhmkorff, p. 46.

chief until their surfaces gave a uniform flush when breathed on. Between these plates was then placed a piece of hand-bill printed on one side only; pieces of tinfoil rather smaller than the glasses were placed on the outside of each, and these coatings were connected with the secondary terminals of a Ruhmkorff coil. After a few minutes' electrization, the coatings were carefully removed, and the interior surface of the glass when breathed on, showed with great beauty the printed words which had been opposite it, these appearing as though etched on the glass, or having a frosted appearance; even the fibres of the paper were beautifully brought out by the breath, but nothing beyond the margin of the tinfoil.

2. It now occurred to me that I might render these impressions permanent by the use of hydrofluoric acid, and a similar experiment was made, the naked plate of glass after electrization being exposed over a leaden dish containing powdered fluor spar and sulphuric acid, and slightly warmed; the letters came out rather imperfectly, but some creases in the paper were beautifully reproduced.

3. I now cut out of thin white letter-paper the word VOLTA, and placed it between the plates of glass; they were submitted to electrization as before, and the interior surface of one of them, without the paper letters, was subsequently exposed to the hydrofluoric acid vapour; the previously invisible figures came out perfectly, and formed a permanent and perfectly accurate etching of the word Volta, as complete as if it had been done in the usual mode by an etching ground. This of course could be washed and rubbed to any extent without alteration, and the results I have obtained give every promise for those who may pursue this as an art, of producing very beautiful effects, enabling Silhouette designs, or even fine engravings, to be copied on glass, &c.

4. I again electrized a plate in the same manner, and then covered the surface having the invisible image, with iodized collodion, and immersed it in a bath of nitrate of silver (40 grains to the ounce) in a room lighted by a candle, in the usual manner as for a photograph. It was then held opposite a window for a few seconds, and taken back into the darkened room, and on pouring over it a solution of pyrogallic acid, the word Volta, and the border of theg lass beyond the limits of the tinfoil, were darkened, and came out with perfect distinctness, the other parts of the glass having been, as it were, protected by electrization from the action of light; the figures were permanently fixed by a strong solution of hyposulphate of soda.

5. A similar experiment to the last was made, but after fixing the impression, the collodion film was floated off; this contained

the impression as it does with an ordinary photograph; and the glass plates being washed with distilled water and dried, showed no impression when breathed on.

6. An electric impression of the word Volta was well rubbed with a handkerchief, then washed with water and alcohol, then dried; the impression still came out by breathing upon it. Some one of the chemicals used in the collodion process had probably had the effect of removing the figure in Experiment 5, but I have not yet ascertained to which this removal is due.

7. Letters cut in tinfoil were substituted for those of paper; the effect was the same, but it seemed to me more feeble.

8. A solution of nitrate of silver was poured over the surface of an electrized plate, so as to form a bath on its surface; a rake, formed of ten common pins, was made to touch the glass with its points along the course of the invisible image; the silver was of course precipitated in an arborescent form, and I thought it probable that the lines of deposition might follow the course of the invisible image, but I could not be certain of any such effect, though in one experiment there seemed to be some slight indication of it.

The above experiments were repeated many times, both with positive and negative electricity from the coil. At first I believed I had found a remarkable difference in the effect of the change of direction of the discharge in the cases where hydrofluoric acid was employed; and in two experiments, in which the tinfoil of the upper glass plate was connected with the positive terminal, the impression of the word on the lower side of the upper plate after exposure to the vapour, was polished, while the residue of the plate was frosted; and in two subsequent cases, the upper tinfoil being connected with the negative terminal, the reverse was the case. In subsequent experiments, however, great irregularities took place in this relation, and it seemed to depend on the time of exposure and on slight differences in the distance between the paper letters and the glass, the latter not being brought into perfectly uniform contact with the surfaces of the glass. After the first few experiments I placed a marble paper-weight on the upper glass, and found the effects more uniform and perfect. An electrization for periods of from five to ten minutes, produced the sharpest and clearest effects; when the electrization was prolonged, a blur or second margin gradually appeared, and increased in extent around the outline of the letters, having somewhat the appearance which would have been presented had the paper letters been moist, and the liquid slightly extended itself from their edges over the glass.

When the electrization was thus prolonged, the figures were visible on inclining the glass to the light, without breathing on them, and gave a strong impression of the glass having suffered Phil. Mag. S. 4. Vol. 13. No. 83. Jan. 1857.

F