182 
Dr. Thomson on some of 
If we subtract the ri chromic acid from the 13-32, origi- 
nally present in the chromate of lead, there will remain 12*22, 
which had yielded 9*36 of green oxide. According to this 
analysis chromic acid is composed of 
Protoxide of chromium - 9-36 or 4-9 
Oxygen - - - 2-86 or 1*5 
12*22 
The atomic weight of protoxide of chromium, by this ex- 
periment, comes out 4*9, instead of 5 ; but a few flocks of the 
green oxide of chromium accidentally escaped, and could not 
be weighed. Were we to estimate them at -^th of a grain 
(and I think they amounted to that quantity), the atom of 
protoxide would amount exactly to 5. 
This mode of experimenting is susceptible of considerable 
accuracy. The error was only 2 per cent, and the source of 
it was evident. 
2. 6-62 grains of anhydrous green oxide of chromium 
were heated with hydrate of potash, over a spirit lamp, till 
water ceased to escape. The silver crucible containing the 
mixture was then raised to a red heat, and kept in that tem- 
perature for 15 minutes. The whole was now digested in 
water: 1*49 grains of the anhydrous green oxide still re- 
mained in its original state ; so that by the process, 5*13 
grains of green oxide had been converted into chromic acid. 
The yellow-coloured alkaline solution was saturated with 
acetic acid, and precipitated by acetate of lead. The chromate 
of lead obtained weighed after ignition 20*29 grains, equiva- 
lent to 6*433 grains of chromic acid. 
According to this experiment chromic acid is composed of 
