\ 
the compounds of chromium. 185 
12*1 grains of this dry acid were put into a green glass 
tube, shut at one end, and bent as the tube 
( A) in the margin, so as to serve the pur- 
pose of a retort. The open end ascended 
to nearly the top of a glass jar (B) stand- 
ing in the mercurial trough (C), and 
containing 3 cubic inches of air over a 
column of mercury 3-6 inches long. While mL 
the barometer stood at 30-13 inches, and the thermometer at 
52 0 . The flame of a spirit lamp being applied to the end of the 
tube (A), containing the acid, fusion took place, and water 
was given out. A little of the acid assumed the form of 
vapour, and was deposited in the upper part of the tube. As 
soon as the tube became red hot, the acid began to give out 
oxygen gas. The heat was continued till all evolution of 
gas was at an end, and till the acid in the tube had assumed 
a fine green colour. 
The oxygen gas evolved when reduced to the temperature 
of 6 o°, under a pressure of 30 inches of mercury, was 5-634 
cubic inches, the weight of which is 1-909 grain. The real 
acid heated amounted to 10-765 grains. According to this 
experiment, chromic acid is a compound of 
Protoxide - - 8-8 56 or 6-95 
Oxygen - - 1-909 or 1*5 
10-765 
What makes this result so erroneous, is partly the impos- 
sibility of determining the quantity of chromic acid which had 
sublimed unaltered ; but chiefly the impracticability of re- 
ducing the acid completely to the state of protoxide by the 
application of the greatest heat which the glass could bear 
MDCCCXXVII. B b 
