of Ammonia and Chromium. 193 



vol. xlv. p. 299), he gives a full account of the deportment of 

 chromium with ammonia, and mentions the sulphuric acid com- 

 pound I have above alluded to, but ascribes its production to a 

 totally different cause from that which I have assigned. 



His mode of proceeding was as follows : — 1. He evaporated a 

 solution of chrome-alum with a small quantity of oil of vitriol to 

 a small bulk, and dropped this mixture into an excess of con- 

 centrated ammonia. ("By this means," he says, "less than 0"3 

 per cent, of chromic oxide remains undissolved.") 2. He drop- 

 ped a solution of chrome- alum into ammonia (producing a 

 greyish-violet precipitate), and dissolved this unwashed preci- 

 pitate in sulphuric acid. 



These solutions have a wine-red colour, and are the same as 

 the acid and alkaline modifications I have mentioned above. 

 Hertwig, however, does not recognize that the coloured solution 

 of the chromic oxide by both the first and second method is owing 

 to the same cause, viz, the formation of a double salt of sesqui- 

 oxide of chromium and ammonia with sulphuric acid, but be- 

 lieves that the first instance is a simple case of the solution of 

 the chromic oxide by the concentrated alkali. Of the second 

 case he gives no explanation. He points out also the necessity 

 of a purple modification of the green hydrate of chromic oxide 

 being formed previous to any solution in ammonia, but overlooks 

 entirely the fact, that without the presence of an acid this modi- 

 fication is never produced, and when obtained is as insoluble as 

 the green variety. 



I have mentioned above that different tints are produced 

 according to the acids used for the solution of the chromic 

 oxide in ammonia; they are as follow: — With nitric acid, rose- 

 pink ; with hydrochloric acid, crimson ; with sulphuric acid, a 

 rich port-wine colour. 



Of the ultimate composition of these compounds I have no 

 means of judging, since every attempt to obtain them in the solid 

 state has hitherto failed. If exposed to the air for any length of 

 time, they decompose into the nitrates and sulphates of chromic 

 oxide and ammonia respectively; the hydrochloric acid compound 

 into chloride of ammonia and chloride of chromium : to a mix- 

 ture of this kind the addition of ammonia and hydrochloric acid 

 will restore the colour. When heated, ammonia is given off and 

 a violet powder precipitated; this also is rcdissolved by ammonia 

 with restoration of the colour. None of these compounds are 

 affected by concenti*ated solutions of potassa or soda in the cold ; 

 but when heated they are readily decomposed, and on cooling, 

 the green sesquioxide of chromium is precipitated. 



