324 Dr. Thomson on [Nov. 



22. SulphcUe of Magnesia. — 0. 



23. Alum. — A copious white precipitate. 



24. Muiiate of Alumina.— A copious gelatinous precipitate, 

 which speedily diminishes in bulk, and becomes crystalline. 



25. Frussiate of Potash. — 0. 



26. Tincture of' Nutgalls. — A very copious chocolate-brown 

 precipitate. Becomes dirty-orange, when dried. 



27. Hydriodate of Zinc. — A copious yellow precipitate. 



28. Alcohol. — The chromate of potash is immediately precipi- 

 tated in small crystals. 



Such are the properties of this salt as far as I have determined 

 them. To analyze it, I find the best reagent is nitrate of lead, 

 which it is well known throws down the chromic acid united to 

 the oxide of lead in the form of an intensely orange powder. 

 Nitrate of mercury indeed is fully as sensible. I find that when 

 1 gr. of chromate of potash is dissolved in 40,000 grs. of water, 

 a very considerable precipitate is obtained when we drop into 

 the solution either nitrate of mercury, or nitrate of lead. When 

 the solution of the chromate of potash is so much diluted, the 

 precipitate by nitrate of mercury is reddish-yellow, and that by 

 nitrate of lead greenish-yellow. Nitrate of silver is a less sensible 

 reagent for discovering chromic acid ; for it occasions no preci- 

 pitate in a solution of 1 gr. of chromate of potash in 40,000 grs. 

 of water; but 1 gr. of the chromate dissolved in 20,000 grs. of 

 water may be detected by this reagent. Nitrate of barytes is 

 about as sensible as nitrate of silver, producing like it a precipi- 

 tate in a solution of 1 gr. of chromate of potash in 20,000 grs. of 

 water, ^^hile this solution is neither acted upon by nitrate of 

 lime, nor muriate of strontian. 1 have tried to analyze chromate 

 of potash both by means of nitrate of lead, nitrate of mercury, 

 and nitrate of barytes ; but the I'esults obtained by means of 

 nitrate of lead were on the whole the most satisfactory. The 

 method which I found to answer best v, as the following : 



Expose the chromate of potash to be subjected to analysis to 

 a red heat. Then weigh out 20 grs. of it, and dissolve them in 

 distilled water. Drop into this solution a solution of crystal- 

 lized nitrate of lead in water as long as any precipitate continues 

 to fall. Separate this precipitate, wash it well, dry it, and then 

 expose it for an hour to a heat of 400°. Then weigh it ; i-^, or 

 0"317, of this precipitate is chromic acid. Therefore multiply 

 the weight of the precipitate by 0-317, the product indicates the 

 weight of chromic acid in 20 grs. of chromate of potash. If the 

 experiment be rightly performed, the chromate of lead from 20 

 grs. of chromate of potash will weigh 32*8 gr. Now -i-f of 32*8 

 is 10"4. Thus the chromic acid in 20 grs. of chromate of potash 

 is 10*4 gr. ; therefore 100 grs. of the salt must contain 10*4 x 5 

 = 52 grs. The only other constituent being potash, it is obvious 

 that chromate of potash is composed of 



