2l8 
Dr. Thomson on some of 
2 grains. But as 29*75 grains of the salt contain 27*5 grains 
of the anhydrous salts, it is plain that the 2*25 grains wanting 
to make up the total must be water. 
VI. Chromates. 
In my “ First Principles of Chemistry ” I have given an 
account of twenty species of chromates, or combinations of 
chromic acid with a base. I shall take the present oppor¬ 
tunity to give a short account of a few more of these salts 
which I have had occasion to examine, but without pretending 
to exhaust the subject. 
1. Perchromates of Iron. 
As the protoxide of iron has the property of reducing 
chromic acid to protoxide, it is evident that no protochromate 
can exist; but I thought it worth while to try whether I 
could not obtain a combination of chromic acid and peroxide 
of iron. 35 grains of iron were dissolved in dilute nitric acid, 
the solution was heated on the sand-bath and evaporated to 
dryness. The red-coloured residue, consisting chiefly of 
peroxide of iron, was redissolved by means of muriatic acid, 
and the muriatic solution was made as neutral as possible; 
first, by evaporation, and then by adding a few drops of 
ammonia. 
125 grains of chromate of potash in crystals were dis¬ 
solved in water. 
These two solutions being mixed together, a dark reddish 
brown precipitate fell in large flocks. This precipitate w^as 
collected on the filter, edulcorated, and dried on the sand- 
bath. The brown matter thus collected weighed 30*45 
