1 66 Elements of Mineralogy. 



in the folution, as the excefs of the Pruffian 

 alkali, which muft neceflarily be added, to b& 

 certain that all the iron is precipitated, never 

 fails to re-diflblve a portion of the Pruffian 

 blue which thus remains in the liquor, and 

 cannot be got rid off. Hence the method I 

 ufe is as follows : firft, I prepare the Pruffian 

 alkali after the manner of Mr. Bergman, by 

 digefting and boiling a pure 'alkaline folution 

 over Pruffian blue, until the alkali no longer 

 effervefces with acids, nor precipitates a folu- 

 tion of nitrous felenite, or any other Earth, 

 except the barytes : I even make it a little 

 ilronger ; for if it be barely faturated with the 

 tinging matter, it foon fpoils and precipitates 

 Other Earths, the tinging matter evaporating. 

 I next examine how mucih of ,this alkali is 

 neceflary to precipitate i gr. of iron from its 

 folution in dilute vitriolic or marine acid, and 

 I mark this on the label of the bottle that 

 contains the alkali. I now come to the ap* 

 plication, 



The folution of the earths being weighed 

 I take 100 grains of it, and on thefe I gra- 

 dually pour the Pruffian alkali (a portion of 

 which is alfo previoufly weighed) until all 

 the iron, or ponderous Earth and iron, is pre- 

 cipitated; the weight of the alkali ufed, gives 

 that of the iron contained in 100 grains of 

 the folution, and the quantity contained in 



rpg 



I 



