Chemical Constitution of Saline Solutions. 281 



Notes on the Salts examined. 



Chromium Chlorides. — Ghi'omium forms at least three chlorides, the auliydrous, 

 Cr2ClG, a lilac-coloured compound, is insoluble in water, but it slowly undergoes 

 hydration, and then foi'ms a green solution similar to that obtained when chromium 

 hydroxide is dissolved in hydrochloric acid. But there are at least two varieties 

 of the green chromic chloride, one precipitable by ammonium oxalate, the other 

 not. 



Chromium Sidphate. — The following experiment shows that the green compound 

 is formed when the violet solution is heated, 0-5 grs. of violet chromic sulphate 

 were dissolved in 4 c.c. of water. A like quantity of the salt was converted into 

 the green compound Cr2(S04)3'5H20 by heating it in a water bath. It was 

 dissolved in water, and made uj) to the same volume as the first solution. The two 

 solutions were then examined in test-tubes of the same diameter. 



Chromic Nitrate, Cr3(N03)'9H20.- — Ordway prepared this salt,* but it was 

 also prepared and examined by me independently. Blue chromium hydroxide 

 precijDitated from chrome alum by ammonia was dissolved in nitric acid as long as 

 the liquid remained blue. If it was seen to change to green, a little moi'e nitric 

 acid was added. The liquid was evaporated in an air-pump bell-jar, but it was 

 difficult to obtain a crop of crystals. After remaining for some time in a desiccator, 

 a number of octahedral crystals were deposited. A second and larger quantity of 

 liquid yielded only an ounce weight of crystals in the course of two years. The 

 mother liquor frequently becomes green and uncrystallizable after it has arrived at 

 such a stage of concentration. The salt itself is of a deep violet colour ; it becomes 

 green at 36°, as Ordway has stated. It loses water of crystallization in a dry 

 atmosphere and also at a very gentle heat. 



The remarkable change in the spectrum on heating is shown in figs. 1 and 2, 

 plate XX. B. The green rays alone were transmitted by the hot solution, but 

 only a narrow and sharply selected band of tliese. 



After cooling and keeping for some days the liquid had resumed its first 

 condition and gave the original spectrum. 



This substance is evidently unchanged by heating to 100°. , 



Experiments were then made with equal weights of the violet chromic nitrate, 

 one portion being dissolved in water at 16°, the other being heated to 100°, and 

 then dissolved in cold water {t — 18°), the two solutions being made up to equal 

 volumes. 



The spectrum is practically the same as that of the nitrate solution at 100°; hence, 

 the violet salt is converted into the green modification by heating the solution 

 to 100°. 



* Phil. Mag., vol. 36, p. 205. 



