PHYSICAL AND LITERARY, li 



and the original weight of the filter, 

 gives you the weight of unfoluble 

 parts contained in the marie under 

 examination. This being known, the 

 proportion of calcarious earth in the 

 fame marie is evident. The propor-* 

 tions of clay and fand in it are dif^ 

 covered by fubjecling the refiduwji to a 

 proper elutriation. This operation is very 

 fimple, and performed thus : Having 

 weighed the dry refidne, mix and fhake it 

 well with a fufiicient quantity of water. 

 After allowing a little time for the fubfi-' 

 dence of the groffer parts, let the water, 

 tvith the fineft particles of clay fufpended 

 in it, be gently poured off. ;When this 

 is done, add more water ta the remain- 

 der, and after fuiEcient mixture and fub- 

 fidence, pour that off iikewife. In the 

 fame manner, repeat the operation again- 

 and again^ till the water came over per- 

 fedlly pure. The fubftance which thenf 

 Remains is fand^ mixed, perhaps, with 

 fome flakes of talc ; and whatever this^ 

 fubftance wants of the weight of the refi- 

 due employed, is the weight of pure clay 



- carried 



