242 On the Combination of Chrome with Sulphuric Acid. 
flying completely out of the mercury; when the contact being 
thus broken, it fell by its own gravity to be again projected, and 
so on, as long as the action of the battery lasted. 
The name of the young man alluded to above, is James Marsh, 
a very ingenious workman employed in the laboratory of the 
Royal Arsenal, who has constructed for me my calorimotor, and 
most of the other apparatus I have had occasion for in my ex- 
periments. It is much to be regretted that he is not in a situa- 
tion to allow of a further and more profitable exercise of his in- 
genuity. 
I remain, dear sir, yours very truly, 
Royal Military Academy, P. BaRLow. 
March 13, 1822. 
LII. On the Combination of Chrome with Sulphuric Acid. By 
Gay-Lussac*. 
Wauex dilute sulphuric acid in considerable excess is boiled upon 
chromate of lead or of barytes, the chromic acid is never pure. 
It always retains much sulphuric acid, even when ten times as 
much acid is used as is required to decompose the chromate em- 
ployed. When the liquid containing the two acids is submitted 
to successive evaporations, it totally crystallizes in small quadran- 
gular prisms of a deep red: if the concentration and heat are 
excessive, the chromic acid is partially decomposed, and the re- 
sult is sulphate of green oxide of chrome. ‘These crystals are 
very soluble in water, even deliquescent, and consist of an atom 
of sulphuric acid and an atom of chromic acid. They are ana- 
lysed thus: Boil them with a mixture of muriatic acid and alco- 
hol, to decompose the chromic acid, and change it to green 
oxide; then divide the liquid into two equal parts, precipitate 
one of them with muriate of barytes, to estimate the sulphuric 
acid; and the other with ammonia, to obtain the oxide of chrome; 
whence the quantity of chromic acid may be inferred. This 
compound of the two acids may be obtained at once by mixing 
them in a concentrated state; when a red precipitate immediately 
fallsdown. Nitric acid does not appear to adhere with any force 
to ie acid, nor do they crystallize together, as with sulphuric 
acid. 
The compound of chromic and sulphuric acid is readily dis- 
solved in alcohol; but if the solution is concentrated, the reci- 
procal action of these. substances takes place with a violence al- 
most explosive. The chromic acid passes to the state of green 
oxide, and the liquid acquires a peculiar ethereous smell, similar 
* From the Annales de Chimie et de Physique. 
to 
