192 'Mv. C. W. Vincent on certain Double Compounds 



uniformly found that when a well-washed precipitate was used, 

 it was perfectly insoluble in ammonia. I now passed a small 

 quantity of hydrochloric acid through the filter containing the 

 sesquioxide of chromium, so as to moisten it with the acid, and 

 then again submitted it to treatment with concentrated ammonia, 

 heating the mixture for a few minutes. The precipitate during 

 this time changed its colour from green to a greyish-blue ; and 

 on filtering, I found the ammonia had acquired a distinctly red 

 hue. I then added a rather larger quantity of hydrochloric acid 

 to the precipitate than was sufficient for its complete solution, 

 reprecipitated mth ammonia, and filtered. The bulk of the pre- 

 cipitate I observed to be much less, and the colour more intense 

 than previously. By continuing this kind of treatment with 

 hydrochloric acid and ammonia alternately, I at length reached 

 a point where the ammonia no longer produced a precipitate, 

 but changed the colour of the liquid from a dingy green to a 

 beautiful crimson colour. "W-lien I substituted nitric or sulphuric 

 acid for the hydrochloric acid in the foregoing experiment, I ob- 

 tained precisely similar eflPects; but the respective tints of the 

 solutions produced were different. 



From the behaviour of the chromic oxide in the preceding 

 experiment, I was led to infer the existence of a double salt of 

 ammonia and chromium in the solution, to which the colour is 

 due, and not, as has been supposed, to the mere solution of the 

 chromic oxide in ammonia. 



The following is a more convenient method of producing these 

 compounds* : — A concentrated solution of ammonia is gently 

 heated; to this a solution of the sesquichloride of chromium, 

 acidulated with a small quantity of hydrochloric acid, is added 

 drop by drop. A green precipitate immediately falls down, 

 which on standing gradually changes to a blue, violet, and lastly 

 indigo colour. On the addition of hydrochloric acid to this pre- 

 cipitate, it readily dissolves, with the formation of the beautiful 

 crimson colour above adverted to. 



In this last experiment I have used an acid as a direct means 

 for the production of the coloured solutions. By this plan the 

 colours are much more intense than when prepared by the first 

 method ; in other respects, however, they are perfectly identical 

 with them. It appears to me, therefore, a strong corroboration 

 of the idea, that it is to the formation of a double salt that the 

 colour is due. 



In an elaborate paper by Hertwig {Annalen der Pharmacie, 



* I have used the chloride of chromium and hydrochloric acid as an 

 example ; but with the nitrate of chromic oxide and nitric acid, or with the 

 sulphate and sulphuric acid, the same course is jiursued when it is desii'ed 

 to produce the compounds corresponding to these several acids. 



