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the acid with which it is to be combined, or by dropping its fo- 

 •lution in a weaker acid into the acid with which it is intended 

 to be combined. In the ift mode of combination the faturatlon 

 is fcarce ever complete, becaufe the new compound in many cafes 

 precipitates before it is fully faturated, and even though there 

 fhould be an excefs of the acid to be combined in the liquor, yet 

 the inferior part of the precipitate feldom receives it, being fliel- 

 tered by the fuperior, and becaufe its affinity to its laft comple- 

 ment of acid is much weaker than that to its mean proportion of 

 acid. 



But in the 2d or 3d mode of addition, the earth being fur- 

 rounded by the acid with which it is to be combined, and thus 

 expofing a greater furface, takes up more of it and even fre- 

 quently an excefs, as I have often experienced. 



This explains the diflPerence which may be obferved in the ex- 

 periments I fhall now ftate : 



ift. Dr. Withering having made a folution of 100 parts native 

 aerated barytas in muriatic acid, dropped vitriolic acid into it until 

 a precipitation ceafed to appear ; this artificial barofelenile weighed 

 117 grs. Phil. Tranf. ibid. 405. Now this native barytes con- 

 tained but 78, 6 of pure barytic earth, as he had proved in a 

 former experiment; therefore 78,6 of barytic earth took up as 

 much real vitriolic acid as raifed its weight to 1 17 grs. namely 38,4 

 grs. ; and if 1 17 grs. barofelenite contain 38,4 grs. of vitriolic acid, 

 100 parts barofelenite muft contain 32,8. 



Vol. VII. D d So 



