224 
Dr. Thomson on some of 
i atom chromate of potash 
i atom chromate of soda 
| atom of water 
12*5 
io *5 
0-5625 
23*5625 
Though the water happens to amount almost exactly to 
half an atom, it is probable from the decrepitating property 
of the salt, that it is only mechanically lodged between the 
plates of the crystals. 
This compound salt differs exceedingly from a mere com- 
bination of chromate of potash and chromate of soda. Chro- 
mate of potash is anhydrous, but chromate of soda contains 
12 atoms of water. All this water is excluded when the 
compound salt is formed. 
Bichromate of potash, when digested over carbonate of 
magnesia, dissolves it with effervescence, and gradually 
assumes the yellow colour of the neutral chromates. This 
happens when the excess of acid in the bichromate is saturated 
with magnesia ; so that 19 grains of bichromate of potash 
just dissolve 2-5 grains of magnesia. When the liquid is 
concentrated, it deposites a crust of the compound salt at the 
bottom of the vessel. This crust consists of a congeries of 
small hard crystals, so closely interwoven that it is scarcely 
possible to make out the shape. Some of the facets appear 
to belong to octahedrons, while others are more like faces of 
a prism, seemingly four sided, and rectangular. The colour is 
a fine yellow, and the salt is not altered by exposure to the air. 
The taste is bitter and unpleasant. The salt is pretty soluble in 
5. Potash-chromate of Magnesia. 
