i8o Dr. Thomson on some of 
Green oxide of chromium - 5*589 
Protoxide of lead - - 18*216 
23*805 
which exceeds the weight of the matter employed by 0*745 
grains. The oxygen in 18*216 grains of protoxide of lead 
amounts to 1*301. Of this only 0*556 gr. existed in the 
black powder. From this experiment it appears that ^-ths of 
the lead were reduced to the metallic state, while ^-ths still 
remained in their original state of protoxide. 
The lead here exceeds the quantity which would have 
been obtained, had I been able to analyse the whole altered 
chromate of lead. The true proportions would have been 
Green oxide of chromium - 5*589 
Protoxide of lead - - 15*645 
The excess of 2|- grains was owing to this; I took the 
bottom of the cake, because the upper part was not free from 
charcoal; and the great weight of the reduced lead occa¬ 
sioned an excess of it to be found in the bottom of the cake. 
Now the oxygen in 15*645 grains of protoxide of lead is 
1*117, of which 0-556 gr. is very nearly the half. From this 
we see, that when chromate of lead is exposed to a white 
heat in a charcoal crucible, the chromic acid is totally con¬ 
verted into green oxide, while half the protoxide is reduced 
to metallic lead. The black matter formed is a compound of 
2 atoms green oxide of chromium - 10 
1 atom protoxide of lead - - - 14 
42 grains of chromate of lead, in powder, were put into a 
green glass tube, which by means of a charcoal fire was kept 
at a cherry red heat, while a current of dry hydrogen gas 
