1820.] Chromate and Bichromate of Putash. 325 



Chromic acid 52 or o-o 



Potash _4S 6-0 



100 



If any of my readers wish to repeat this analysi"3, it will be . 

 necessary for him to make sure of the purity of his chromate of 

 potash, which is by no means easy. I repeated the analysis four 

 times with crystals of chromate of potash which I had prepared 

 on purpose ; but the results never agreed exactly with each 

 other. Nor was 1 able to obtain constant results till I formed 

 the chromate of potash from the bichromate, by dissolving that 

 salt in water, and adding with iuhnite care exactly the quantity 

 of potash necessary to neutralize the chromic acid. Uo\v I 

 ascertained that the exact weight of an atom of^chromic acid is 

 6"-5, and the exact composition of chromate of potash, I shall 

 exp'lain in a paper which will be found immediately after the 

 present. The introduction of the necessary details would lead 

 us too far at present from the subject in hand. 



The only other person who has made experiments, as far as 1 

 know, to determine the weight of an atom of chromic acid is 

 Prof. Berzelius. According to him, an atom of chronuum 

 weighs 7-0364, and an atom of chromic acid 39-1092. _ These 

 numbers differ enormously from mine ; but the composition of 

 chromate of potash resulting from his method does not differ 

 much from the analysis which I have just given. According to 

 Berzelius, it is composed of 



Chromic acid 52*49 



Potash 47-51 



100-00 



I shall demonstrate in the subsequent article, I trust, to the 

 satisfaction of chemists, that the numbers which 1 have given 

 are the true ones. I think it not unlikely that Berzelius tailed 

 in consequence of the presence of a small quantity of saltpetre 

 remaining mixed or united to his chromate of potash. This at 

 least was°the case with the chromate of potash \vhich I attempted 

 to analyze when I began my experiments on this salt. 



2. Bichromate of Potash".— This, is perhaps the most beautiful 

 salt which chemists have hitherto discovered. It has an intense 

 orange-red colour, and crystalhzes sometimes in large rectangu- 

 lar or square tables, with bevelled edges, the inclination of the 

 bevelled faces to the corresponding face of the prism about 120°, 

 sometimes in flat rectangular four-sided prisms, terminated ux 

 some cases by a face perpendicular to the axis of the prism, m 

 some cases by a four-sided pyramid, and in others by a dihedral 

 summit. I have obtained the tables nearly an inch in length, 



