176 
Dr. Thomson on some of 
Green oxide 5 
Oxygen - - 1*5 
and 5 is the atomic weight of green oxide of chromium. 
The whole protoxide of iron used in this experiment, when 
converted into peroxide, became 15 grains, and the 6-5 
chromic acid, when reduced to green oxide, became 5 grains. 
Hence the total weight of the oxides of iron and chromium 
ought to amount to 20 grains. 
Now the buff-coloured precipitate was - 16-81 
In solution [P eroxideofiron ' * ^ 
L green oxide of chromium - 166 
i9‘84 
Loss - - - - - - o* 16 
The cause of this loss became evident when the residual 
liquid was concentrated to a few drops. It should have con- 
tained sulphuric acid, potash, and the alkali employed to 
throw down the green oxide, and nothing else. But after the 
concentration had made considerable progress, the liquid 
assumed a perceptibly yellow tinge, showing that it still 
retained a small portion of chromic acid. The colouring 
powers of this acid are so great, that a very minute portion 
of it becomes visible. The protosulphate of iron, though I 
had been at great pains to have it as pure as possible, was 
not quite free from all admixture of peroxide. For when a 
crystal of it was put into a solution of prussiate of potash, a 
green colour was immediately struck. This incipient perox- 
idizement of the iron rendering the quantity employed insuffi- 
cient to reduce the whole chromic acid, a minute portion still 
remained in the state of chromic acid. 
