170 
Dr. Thomson on some of 
stituting a bihydrosulphuret of soda, the whole oxygen of 
the sulphurous acid would have been converted into water, 
and the hyposulphurous acid would have contained no oxygen 
at all. 
Such is the composition of the hyposulphurous acid of 
Herschel. I verified this constitution by the direct analysis 
of several hyposulphites ; the most easily analyzed of which 
were the hyposulphites of barytes and of lead.* 
The reader being now aware of the composition of hypo- 
sulphurous acid, will be prepared for the theoretical explana- 
tion of the decomposition of chromic acid by sulphuretted 
hydrogen gas. 
Let us suppose a solution of 125 grains of chromate of 
potash, containing 65 grains (equivalent to 10 atoms) of 
chromic acid. To reduce this to green oxide, 5 atoms of sul- 
phuretted hydrogen gas are requisite, containing 
5 atoms sulphur, 
5 atoms hydrogen. 
The sulphur is converted into sulphurous acid and must 
combine with - - - 10 atoms oxygen, 
The hydrogen unites to 5 atoms oxygen, 
forming water. 15 
Thus the 10 atoms chromic acid, in order to become green 
oxide, must part with 1 5 atoms of oxygen ; or ( which is the 
same thing) 1 atom of chromic acid is converted into green 
oxide when it is deprived of 1*5 atom oxygen. But chromic 
• There is another acid which exists, composed of i atom sulphur -|- i atom 
oxygen. Its atomic weight is 3. It may be distinguished by the name of sub- 
sulphurous acid. 
