206- 
Dr. Thomson on some of 
But no confidence can be put in this analysis ; for nitrate of 
ammonia is apt to be decomposed by too high a temperature ; 
and the salt had been left for 24 hours upon the sand bath. 
When nitrate of chromium is exposed to heat in a platinum 
crucible, it melts, and copious fumes of nitrous gas are given 
off. When the heat is continued till these fumes cease to be 
disengaged, a great part of the green oxide is converted 
into chromic acid. The residue is soluble in water, and 
forms a dark brown acid liquid, consisting of chromic acid 
holding green oxide in solution. This affords an easy way 
of getting pure chromic acid ; for the small quantity of green 
oxide present will not interfere with the application of the 
chromic acid to most purposes for which chemists are likely 
to employ it. When we prepare chromic acid by Vauquelin's 
method, besides the expence of the process, it is by no means 
easy to free the acid from all traces of sulphuric acid. 
3. Sulphate of chromium . 
I mentioned before, that when hydrated green oxide of 
chromium is dried in the open air, without the application of 
artificial heat, it contains yfths of its weight of water. When 
dried on the sand bath, it retains about half its weight of 
water. The last of these is darker coloured than the first. 
It is much easier to dissolve the former of these hydrates in 
sulphuric acid than the latter. With the former we can 
obtain a neutral salt ; but I never was able to saturate sul- 
phuric acid by digesting it over the latter hydrate. 
Sulphate of chromium, while in solution, forms a dark 
green opaque liquid, having a sweet, and slightly acidulous 
taste. Like all the salts of chromium it reddens vegetable 
