The eligibility of a liquid in the construction of a battery will, of course, be much influenced by its conducting power: and it would at first sight appear that this might be easily determined by placing the different liquids, in succession, in the same voltameter, or experimental cell, and transmitting through them a constant current of known power; measuring the retarding influence of each by another voltameter charged in the usual way with dilute sulphuric acid, and included in the same circuit. The following are the results of some experiments so performed. The current from ten cells of the small constant battery was employed:

9.2 7.9

Dilute sulphuric acid saturated with sulphate of copper

Saturated solution of copper

Bichcromate of potassa, specific gravity 1·050....

5.6

Let us however consider these results with reference to the formula. The expression for the arrangement used becomes

in which e' signifies the contrary electromotive force, introduced by the accumulation of the ions on the plates of the voltameter containing the liquid tested, and the resistance offered by the same. It is clear that we cannot estimate r', which we are attempting, unless e' is known or remains constant: now e' is not constant, since with nitric acid it vanishes probably altogether, and varies with each of the other substances employed.

If chloride of platinum were not too expensive to allow of its being employed as the exterior part of the electrolyte in contact with a platinum conducting plate, e', or the contrary electromotive force would be wholly annihilated, as nothing but platinum would be thrown down upon the platinum, and it would constitute the most perfect possible arrangement, but would not much, if anything, exceed the efficiency of nitric acid.

In the ntiric acid battery the electromotive force is nearly double

that of the sulphate of copper arrangement, and consequently one cell of that construction is capable of effecting the decomposition of dilute sulphuric acid. It is evident that a similar series of calculations might be made for this battery and others in which different electrolytes are employed; R varying as before with the size and distance of the plates.

I have, however, done enough for the accomplishment of my present purpose; but must not conclude without expressing my obligations to my friend Dr. Miller, for his able assistance, both in the performance of the experiments and the calculations of the results which I have now the pleasure of communicating to you. I remain, my dear Faraday, Very faithfully yours,

King's College, 12th April, 1842.

J. F. DANIell.

Contributions to the Chemical History of Palladium and Platinum. By ROBERT KANE, M.D., M.R.I.A. Communicated by FRANCIS BAILY, Esq., V.P.R.S., &c., &c., &c.*

NOTWITHSTANDING the attention which has been paid to the properties of the noble metals by the chemists who have made their compounds an object of study, their history is yet very far from the state of completeness, to which so many departments of inorganic chemistry have recently been brought. The researches hitherto made have had for their objects generally, either the more direct or certain extraction of the metal from the state of combination in which it naturally exists, or the examination of some few compounds, which were remarkable for their beauty or facility of production, or important from their applications. But the general history of these metals has as yet been but imperfectly studied, as may be seen by reference to the meagre account of their salts and other compounds, which even the most extended systematic works present. It is my object in this and in some subsequent papers, to examine specially into the composition and properties of the compounds of palladium, platinum, and gold, and to endeavour to ascertain how far they agree, and in what they differ, as to the laws of combination to which these compounds are subjected. As this paper may be considered but as the commencement of this work, the general bearings of which may change according to the progress of our knowledge, I shall not attempt to give to it any systematic form, or to arrange the bodies to be described in any order or classification, except that all the compounds of the same metal will in each memoir be described together.

It is my duty, at this moment, to express my sincere gratitude and thanks to the Council of the Royal Society, which most kindly

• From the "Philosophical Transactions," for 1842.

placed in my hands, for the purposes of these investigations, a portion of the palladium that had been bequeathed to the society by its illustrious discoverer, to be used in the advancement of science. Should the results I have obtained, in endeavouring to extend and render more accurate our knowledge of the compounds of that remarkable metal, appear such as to justify that appropriation, for which, when made, I feel I had little claim, I shall be fully rewarded for the time and labour they have required, and use my best efforts to extend them by subsequent researches.

SECTION 1.-PALLADIUM COMPOUNDS.

Oxides of Palladium.

It has been long established that palladium combines with oxygen, at least in two proportions, forming the protoxide, which is the basis of its ordinary salts, and the deutoxide, which appears to be analogous to the deutoxide of platinum, and to react in many cases as an acid. To this last body I have not hitherto directed much attention, but some properties of the protoxide which I have noticed appear not unworthy of being described.

The protoxide of palladium is best prepared by the decomposition of the protochloride, by means of a solution of carbonate of soda in excess. The precipitate which first forms is light coloured, but it soon becomes darker, carbonic acid gas is disengaged, and finally an ochrey brown powder falls, which, by drying, becomes dark brown. The precipitation is in this case by no means perfect, the liquor is coloured yellow by traces of the metal dissolved, and the precipitate retains with obstinacy traces of the alkali, from which, however, it may be freed by washing.

When this substance is heated, it first evolves water and then oxygen, leaving a black powder, to the nature of which I shall recur. By a very high temperature (full white heat) it is totally reduced to the metallic state.

The analyses of this hydrated oxide, when first performed, led to very irreconcilable results, owing to two circumstances: 1st, that the oxide of palladium is by no means so easily reduced to the metallic state by the mere agency of heat as has been supposed; and 2nd, that although the precipitation of the hydrated oxide is accompanied with the disengagement of much carbonic acid, yet the precipitate always contains some traces of that acid: it effervesces very distinctly when dissolved in dilute muriatic acid, and is in fact a highly basic carbonate of palladium, rather than a true hydrated protoxide. The following details of the experiments made as to its composition, will place these circumstances in evidence.

A. 53-524 grains of a specimen which had been carefully washed until the liquors ceased to react alkaline, were gently heated over the flame of a spirit-lamp, until no more traces of water were evolved. The residue, a jet black powder, of anhydrous oxide,

L

weighed 45.224 grains, or 84.49 per cent., having lost 15.51, apparently only water.

B. 41.102 grains of another portion, similarly treated, gave a dry residue of 34 512 grains, or 83.96 per cent. This was then heated to full redness, and when cold weighed 31 779 grains, or 77.32 per

cent.

C. 72-481 grains of a specimen prepared at another time gave, when dried until the last traces of water had been driven off, a black powder weighing 61 241 grains, or 84.49 per cent., and at exposure to a red heat was reduced to 56 131 grains, or 77.45 per cent.

D. 56.578 grains of a different specimen gave when dried 48.306 grains, or 85.38 per cent., having lost 14.62 of water. By a red heat it gave off oxygen, and was reduced to 44.846 grains, or 79.43 per cent.

These results placed together for comparison give,

The material (6 to 7 per cent.) expelled by a red heat is oxygen gas, but I found, by trials conducted after the above results were obtained, that neither is all the material expelled by a moderate heat merely water, nor is the residual black powder metallic palladium.

E. To determine the nature of the black powder which remains after the moderate ignition of the oxide, 51.346 grains of it were introduced into a tube of Bohemian glass, and heated in a current of dry hydrogen gas; it became of itself brightly red hot, water was abundantly, almost explosively formed, and the powder assumed at once a gray metallic aspect. It then weighed 47.165 grains, or 91 85 per cent.

F. To control this result another portion of the black powder, obtained from a different portion of oxide, was heated the same way in hydrogen gas. From 46 300 grains there remained 42.952 grains of metal, or 92.72 per cent.

The quantity of oxygen thus shown to be combined with the metal in this black powder is almost exactly half that which the protoxide should contain. It must therefore be considered as suboxide of palladium, at least, provided it be not a mixture of metal and protoxide, which shall be discussed further on. I shall here only compare the experimental results with those given by the formula Pd, Cl for its composition.

The mean quantity of black suboxide obtained by the moderate ignition of the hydrated oxide as already found, is 78.07, and this is shown by the latter experiments to contain 72.60 of metal; excluding therefore for the moment, the question whether anything but water is first driven off, we find that the oxide of palladium may be obtained anhydrous, that by gentle ignition it abandons one half of its oxygen and leaves a black powder, suboxide, which may be totally reduced to the metallic state by violent ignition or by hydrogen gas, at incipient redness.

The mean quantity of suboxide furnished from 100 of dry protoxide, in the above described three analyses, may be thus compared with theory :—

The analysis D alone gives a result much more closely approximating to theory: by it there is

As there were many circumstances which led me to consider it unlikely that the 15.42, the mean quantity of material expelled from the hydrated oxide by a moderated heat, could be entirely water, I determined the real quantity of water present in the following manner. The substance was placed in a tube of Bohemian glass, which at one end was put in connection with an apparatus evolving dry hydrogen gas, and at the other extremity was adapted to a tube containing recently fused chloride of calcium. When the apparatus had been completely filled with hydrogen, the tube containing the oxide of palladium was heated by means of charcoal. Water was evolved and the metal reduced. The current of gas was continued until all water had been carried into the chloride of calcium tube, and the weights were then determined. It was found that from 45 687 grains of the hydrated oxide, there were obtained 33.532 grains of metal, and 11.298 grains of water, giving 73.95 for the former, and 24.74 for the latter per cent.; but of this 24.74, there were formed 12.49 by means of 11.10 of oxygen, which had been combined with the metal, and the water of hydration amounted therefore to but 12.25 per cent.

The difference between the total volatile matter and the water (3.17 per cent.), may be certainly considered as carbonic acid, from the circumstances under which the substance is prepared, and from