266 



THE GEOLOGIST. 



as far as Th^nard was concerned, were yet unknown, and 

 remained to be discovered. We cannot here enter into the 

 details of Dr. Play fair's modus operandi^ and omit them with 

 less regret since he stated, that his ' Memoir on the true 

 Peroxides' will shortly be published. We shall therefore 

 confine ourselves to an enumeration and brief description 

 of the compounds described by him. Peroxide of Copper 

 is of a brownish black colour. It yields oxygen on being 

 dissolved in acids and chlorides. He obtained a hydrate of 

 this compound oxide. By a strong heat this oxide is further 

 decomposed. The next oxide to which the author referred, 

 was the peroxide of iron. His attention had been directed, 

 a few days since, to an ambiguous sentence in the Comtes 

 Rendus,^^ in which M. Fremy appears to have got indications 

 of this oxide, although no description is given of it. Dr. 

 Playfair stated, that if M. Fremy had pubHshed a detailed 

 account of it, of course the priority of pubhcation entitled 

 him to the priority of discovery, but he had seen no such 

 account. The peroxide of iron resembled in character the 

 peroxide of copper. Like that oxide it contained 2 atoms 

 of water, of which it lost H atom in vacuo. The author 

 then described a peroxide of aluminum, which differed from 

 the others in its relations to water. The water in this oxide 

 played a very important part. The oxide was soluble in 

 potash. It could be obtained in a crystallized state and 

 formed compounds of considerable interest, as throwing 

 light upon the constitution of corundum and other minerals. 

 He then announced some new oxides of zinc, the examina- 

 tion of which would shortly be completed. He had kept 

 himself from examining the metallic acids which might be 

 derived from them, as M. Fremy was engaged in their ex- 

 amination, and he had no wish to interfere with the re- 

 searches of that able chemist. The author pointed to the 

 necessity of doubling the atomic weights of the magnesian 



