322 Dr. Thomson on [Xov. 



some weeks in a dry place. A copious crop of yellow-coloured 

 ciystals was obtained in small needles. These were separated, 

 redissolved in water, and crystallized again. By this means 

 crystals of chromate of potash were procured, which, if not abso- 

 lutely pure, were very nearly so. Dissolve any quantity of these 

 crystals in water, add to the solution such a quantity of sulphuric 

 acid that the liquid will have a pretty sour taste. Set the liquid 

 aside for 24 hours, crystals of bichromate of potash will be depo- 

 sited in small regular needles. I shall now describe the proper- 

 ties of these two salts. 



1. Chromate oj' Potash. — The crystals are usually four-sided 

 slender prisms terminated by dihedral summits ; but by slow 

 crystallization they may be obtained in large oblique four-sided 

 prisms terminated at both extremities by four-sided pyramids. 

 The faces of the prism make with each other angles of 110" 

 and 70°. The inclination of a face of the terminal pyramid to 

 the corresponding face of the prism is about 130°. These 

 measurements are to be considered only as approximations ; for 

 the faces of all the crystals which I examined were too rough to 

 admit of the application of the reflecting goniometer. 



The crystals are not in the least altered by exposure to the 

 open air. Their colour is an intense lemon-yellow with a slight 

 shade of orange. The colouring power of this salt is very great. 

 One grain of it dissolved in 40,000 grs. of water forms a solution 

 which has a very perceptible yellow colour. If we dissolve 1 gr. 

 of chromate of potash and 20 grs. of saltpetre in water, and 

 evaporate the solution, we obtain crystals of saltpetre almost as 

 deep-coloured as chromate of potash itself. 



Its taste is cooling, and bitter, and very disagreeable. It 

 continues very long in the mouth. I put into my mouth a small 

 crystal to taste it. Tv/o days elapsed before my mouth lost the 

 disagreeable impression. Indeed for more than 24 hours, every 

 article taken as food appeared to have the very taste of the salt 

 itself. 



It is exceedingly soluble in water. Boiling water indeed seems 

 capable of dissolving almost any quantity of it : 100 grs. of 

 water saturated with it at the temperature of 60° leave, when 

 evaporated to dryness, 32-6 grs. of dry chromate ; so that 100 

 grs. of water at the temperature of 60° dissolve 48-368 grs. of 

 this salt. The consequence of this great solubility is a consider- 

 able difficulty in obtaining the chromate of potash in regular 

 crystals. 



It is insoluble in alcohol. Its specific gravity is 2-6115. It 

 undergoes no change whatever, though kept ever so long, in the 

 temperature of 400°. When exposed to a red heat, it acquires a 

 beautiful crimson colour ; but on coohng becomes of a very- 

 intense lemon-yellow. It cannot be fused by the greatest heat 

 of a common fire. The fusion may be accomplished by means 

 of a wind furnace ; but in that case, the salt acquires a green 



