Chemistry. 359 



ing in the neutral salt, as 16.85 to 17.8. The salt in question appears there- 

 fore to be sesquisulphate of potash, or to consist of 



Theory. Experiment. 

 3 atoms sulphuric acid - 120 53.33 52.45 



2 atoms potash 96 42.66 42.80 



1 atom water - 9 4.00 4.75 



225 

 Or it may be regarded as constituted of 

 2 atoms of sulphate of potash 

 1 atom of liquid sulphuric acid 



225 

 " It is extremely difficult to procure the sesquisulphate of potash free 

 from bisulphate ; and on repeating an attempt to prepare it, as nearly as 

 possible in the mode already described, I procured a large quantity of bi- 

 sulphate and a small proportion of sesquisulphate. I am well aware that 

 different salts are obtainable by using different quantities of water, for the 

 same proportions of acid and base will yield either sulphate, sesquisulphate, 

 or bisulphate of potash ; and I have found that in order to procure bisul- 

 phate, the solution must be much concentrated. But I am unacquainted 

 with the precise circumstance to which the production of sesquisulphate 

 is to be ascribed ; it may perhaps be referrible to some peculiarity of tem- 

 perature in influencing the time of cooling. 



" In the sesquisulphate subjected to analysis, minute crystals of bisul- 

 phate of potash could occasionally be detected; and very pure bisulphate 

 was obtained by evaporating the residual solution after separating the aci- 

 cular salt ; and eventually the solution became extremely acid and ceased 

 to afford crystals of any kind. 



" When a mixture of crystallized bisulphate and sesquisulphate of pot- 

 ash is exposed to the air, while it retains some of the solution from which 

 it was crystallized, a formation of arborescent crystals occurs at the surface. 

 I have not yet collected a sufficient quantity of this salt for examination, 

 but it is probably only sesquisulphate." 



20. Preparation of Pure Oxide of Zinc. By M. Hermann. — It is 

 by no means easy to obtain this substance perfectly pure. The following 

 is M. Hermann's process. Oxide of zinc, or metallic zinc, is to be dis- 

 solved in excess of sulphuric acid, and the solution being filtered, sulphu- 

 retted hydrogen is to be passed through so long as a brown or yellow j.-e- 

 cipitate is formed. Cadmium, lead, or copper being thus separated, an I 

 the solution filtered, it is to be treated with solution of the chloride of 

 lime (bleaching powder) by which the iron and manganese will be sepa- 

 rated. The solution, again filtered, is then to be crystallized in porcelain 

 vessels, by which sulphate of lime is rejected, and another liquor sepa- 

 rated, which usually contains cobalt and nickel. The crystals of sulphate 

 of zinc are to be dissolved in a small quantity of cold water, and the 



