Affinities of 'Earths. 1 3 



XHII of other Earths, for according to the 

 important difcovery of Mr. I? Arcet, they 

 are all rendered fufible by a proper propor- 

 tion of this Earth, though infufible of them- 

 felves. A difcovery which throws the greateft 

 light, not only on mineralogy, but alfo on 

 metallurgy and the arts of vitrifa&ion and> 

 pottery ; now calcareous Earth requires for its 

 fufion half its weight of magnefia, and only 

 \ of its weight of argillaceous Earth, accord- 

 ing to the experiments of Mr. Achard.^ He 

 did not indeed obferve that it had any effet 

 on filiceous Earth, but this appears to be 

 owing to his having ufed too fmall a propor- 

 tion of the calcareous, for Mr. Gerhard^. 

 having expofed filiceous Earth to a violent 

 heat in a crucible of chalk, found it vitrified 

 in the edges where it touched the chalk, but 

 we may infer that this Earth is lefs a&ed 

 upon and more difficultly than the foregoing; 

 even in * the liquid way, calcareous Earth in 

 fome cafes manifefts the fame affinities, thus 

 if Earth of alum perfectly pure be added to 

 lime water, it will precipitate the lime as 

 Mr. Scheele has fhewn in the Memoirs of 

 Stockholm, for 1776, and the precipitate is 

 foluble in the marine acid, which fhews that 

 the precipitation does not arife from any re- 

 mains of the vitriolic acid in the Earth of 



f Mem, Berlin, 1780. J 2 Gefch. Mineral Reich. 



alum. 



