XV. On some of the compounds of chromium. By THOMAS THOMSON, M. D. F. R. S. L. and E. Professor of Chemistry, Glasgow.

Read March 29, 1827.

THE principal object of this Paper is to give an account of a singular combination of chromic acid, and chlorine, which I discovered about a year and a half ago. But as the investigation of this compound led me to a more careful examination of the oxides of chromium, and a more accurate knowledge of their composition than had been hitherto attained, I shall also state the facts which I have ascertained with respect to these bodies. In a Paper inserted in the Annals of Philosophy (1st Series, vol. 16, p. 321), I have shown that the atomic weight of chromic acid is 65. Two other compounds of chromium and oxygen being known, namely, the green oxide, and the brown oxide, I was induced, from analogy, to consider the atomic weight of the former to be 4.5, and that of the latter 5'5; and to make the atom of chromium 3.5. These views were rather favoured by some experiments on the chloride and muriate of chromium, which I have stated in my "First Principles of Chemistry," vol. ii. p. 52. But these experiments deviating more than two per cent from the theoretical number, could not be considered as decisive. How far my former notions on this subject were correct, will appear from the following statements.

I. Chromium.

The metallic chromium, which I employed in the following experiments, had been reduced by Mr. COOPER, of London, who possesses great skill in the management of furnaces. He was kind enough to give me a specimen of this rare metal several years ago; and I take this opportunity of thanking him for a present, which has been of considerable service in my investigations.

Pure metallic chromium is white, with a shade of yellow : it is very brittle, and easily reduced to a fine powder, which still retains the metallic lustre. It is not sensibly attracted by the magnet, even when in very fine powder. Magnetism then is not a property of this metal. The specimen found magnetic by RICHTER, must of course have contained iron. The specific gravity of chromium I found to be 5·093; but the specimen was not quite free from cavities.

Four grains of this metal, previously reduced to an impalpable powder, were boiled for an hour in nitric acid, without perceptible solution. The process was repeated with aqua regia instead of nitric acid, and the flask was left for two days on the sand bath. It was allowed to remain on the cold sand bath from the 3d June to the 18th July, 1826, during a period of uncommonly hot weather, when the thermometer in the shade was repeatedly at 86°. The acid liquid had assumed a green tinge, but the bulk of the powder was not sensibly diminished. The undissolved portion being separated, was found to weigh 3.73 grains: so that only o'27 grain had been dissolved.

From this experiment it appears that acids do not answer

well for converting chromium into an oxide; I therefore had recourse to alkalies. 3'14 grains of metallic chromium in powder, were mixed with a sufficient quantity of hydrate of potash and nitre, in the proportion of about 5 parts of the former to one part of the latter, and kept for 20 minutes in a red heat in a silver crucible by means of a spirit lamp. The water of the hydrate gradually went off, and a reddish yellow matter remained, which was quite liquid while hot, but became solid on cooling. It dissolved completely in water, and the liquid had the usual yellow colour of a solution of chromate of potash. It was neutralized by nitric acid, and then precipitated by acetate of lead. The chromate of lead thrown down, after being washed and dried upon the filter, weighed 168 grains, but was reduced to 16 23 grains when exposed to a red heat over a spirit lamp.

By this process the metallic chromium had been converted into chromic acid. Now chromate of lead is composed of

From this constitution of the salt, it is easy to deduce the weight of chromic acid in 16:23 grains of chromate of lead : it amounts to 5'146 grains.

Thus it appears that 314 grains of chromium, when converted into chromic acid, become 5'146 grains. This gives us 3.966 for the atomic weight of chromium. For

5'146: 5'14 :: 6.5 (atom of chromic acid): 3.966 = atom of chromium.

My stock of metallic chromium being small, I did not

choose to repeat this experiment; but it had been made with such scrupulous attention, that I am satisfied it must approach very near the truth.

As 3.966 differs by less than one per cent from 4, we can scarcely hesitate about adopting 4 as the true atomic weight of metallic chromium. Indeed 4 must be the true number, if we adopt the law of Dr. PROUT, which I have so amply confirmed by numerous examples in my " First Principles of Chemistry," namely, that the atomic weights of all bodies are multiples of 0.125, the atom of hydrogen: for 0·125 ×

324.

This conclusion will be corroborated by the experiments immediately to be detailed, which leave no doubt that the atomic weight of green oxide of chromium is 5. Now, as this oxide is a compound of 1 atom oxygen + 1 atom chromium, the atom of chromium must necessarily be 4. If we allow an error in weighing to have taken place in the preceding experiment to the amount 0.02 grain, the number 4 would come out as the result of the experiment. Now, I need hardly remark to practical chemists, how very difficult it would be to guarantee any similar experiment from so small an error asth of a grain.

II. Green oxide of chromium.

This oxide is always obtained by depriving chromic acid of a portion of its oxygen. Many processes have been followed. When chromate of potash is digested with alcohol or tartaric acid, the chromic acid is pretty speedily converted into green oxide. A solution of chromate of potash speedily lets green oxide fall when a current of sulphurous acid is passed through

the solution.* The same change takes place if we boil a mixture of chromate of potash and muriatic acid together. The addition of a little alcohol greatly promotes the rapidity of this process. The method proposed by BERZELIUS is one of the most tedious and unproductive. He passes a current of sulphuretted hydrogen gas through a solution of chromate of potash, previously acidulated with muriatic acid.

In whatever way the green oxide of chromium is procured, it is always at first in the state of a hydrate. If we dry the precipitate in the open air, it is a greenish blue tasteless light substance, which may or may not contain carbonic acid, according to the re-agent employed in throwing it down. If we dry it on the filter by artificial heat, its colour becomes a good deal darker, and it retains almost exactly half its weight of water. When it is dried in the open air without artificial heat, ths of its weight are water. In this state of hydrate it dissolves readily in almost all acids. A moderate heat expels the water, and leaves the oxide in the state of an exceedingly beautiful green powder, scarcely soluble in any acid whatever. When this green oxide is heated nearly to redness in an open vessel, it generally glows, or becomes intensely red hot, so as to resemble the appearance of burning tinder. This glowing does not always take place, though it is a pretty common phenomenon. It has not yet been determined, upon what this curious property depends.

• To prevent misapprehension, it will be proper to state, that the oxide thrown down by sulphurous acid has a brown colour; but if we dissolve it in muriatic acid, and throw it down by ammonia, the precipitate obtained will be green oxide. It will be shown hereafter, that brown oxide differs from green oxide, merely by retaining a small quantity of chromic acid, with which the green oxide is combined.