174 
Dr. Thomson on some of the 
with a solution of protosulphate of iron. The determination 
of the quantity of protosulphate necessary to disoxygenise a 
given weight of chromic acid, will give us the quantity of 
oxygen which constitutes the difference between chromic acid 
and green oxide. 
It will be necessary to remember that the atomic weight of 
protoxide of iron is 4-5, and that of peroxide 5 ; and that an 
atom of protoxide of iron is converted into peroxide by uniting 
with half an atom of oxygen. We must remember too, that 
crystallized protosulphate of iron is a compound of 
1 atom sulphuric acid - 5 
1 atom protoxide of iron - 4-5 
7 atoms water - - - 7’ 875 
17-375 
Consequently 17*375 of protosulphate of iron constitute 
the equivalent for 1 atom of protoxide of iron. 
If chromic acid be a compound of 
1 atom green oxide - -5 
i± atom oxygen - - 1*5 
6*5 
as the analysis of it by means of sulphuretted hydrogen has 
shown it to be, it is obvious, that in order to reduce an atom 
of chromic acid to green oxide, we must mix it with three 
atoms of protoxide of iron. Now, 52*125 grains of proto- 
sulphate of iron contain the equivalent for 3 atoms of pro- 
toxide, while 12*5 grains of chromate of potash contain the 
equivalent for an atom of chromic acid. These facts being 
understood and remembered, we are prepared for following 
the details of the experiment. 
