Analyfu of Earths and Stones. 165 



tion being thus effe&ed, the refiduum is to 

 be well wafhed, and the wafhings added to 

 the foluiion: the refiduum, -well dried and 

 weighed, gives the weight of filiceous Earth 

 in the compound. 



The folution is next to be examined ; 

 which I will fuppofe to contain the 4 foluble 

 Earths, calcareous, ponderous, magnefia, and 

 argill, and alfo a calx of iron : it always 

 contains an excefs of acid, of which it is in. 

 great meafure deprived by boiling for a con* 

 fiderable time, as both acids are very volatile, 

 and indeed, of the marine none remains but 

 what is combined with the calx of iron, as 

 the nitrous chafes it from the Earths. By get- 

 ting rid of this excefs of acid, lefs alkali will 

 be required for the fucceding precipitation, 

 and lels aerial acid fet loofe which would retain 

 much of the precipitate by re-diflblving it : 

 the folution fhould then be evaporated to 

 about \ a pint. 



The folution being thus prepared, it is 

 ufual to precipitate the calx of iron from it 

 by the Pruffian alkali ; but to this method I 

 have two objections ; i ft< that the ponderous 

 Earth, if any, would alfo be precipitated and 

 confounded in the Pruffian blue ; and 2 d< that 

 this precipitation, befides being exceeding 

 flow, feldom fails of leaving fome iron Hill 

 M 3 in 



