ihc Attractive Powers of Mineral Acids. ^ i 
Metallic lubftances, when free ft from all foreign mixture, 
are obtained either in a reguline ftate, or in that of a calx. 
Thefe calces, if formed by fire, are conftantly combined with 
more or Ids ot the aerial acid, which is very difficultly ex- 
tracted from them, and very loon re-abforbed ; and if formed 
by folution, they as conftantly retain a portion of their folvent 
or precipitant, fo that the precile weight of the really metallic 
part is difficultly afcertained. But though this ffiould eafily be 
effected, ftill they would for the moft part be unfit for my 
purpofe ; becaufe moft of them, when much dephlogifticated, 
are infoluble in fome or all the acids : hence I chofe metals in 
their metallic ftate for the l'ubjeft of my experiments. Thefe 
conlift of fpecjfically different earths and phlogifton, and of 
this they mu ft lofe a part before they can be diffolved in acids ; 
but, befides that which efcapes in an aerial form, much more 
of it, though feparated from the metallic earth, is yet retained 
in the folution by the compound of acid and calx. It is this 
calx, thus differently dephlogifticated by the different acids, 
whole proportion I endeavoured to afeertain. 
The great difficulty that occurred in this inquiry was, that 
of finding the exafl quantity of acid neceflary to faturate the 
metallic lubftances ; for all metallic folutions turn folution of 
litmus red, and confequently contain an exccfs of acid. And 
the reafon is, becaufe the falts, formed by a due proportion of 
metallic calx and acid, are nearly infoluble in liquids that do 
not contain a further quantity of acid ; and in fome cales this 
quantity, and even its proportion to the aqueous part oj the liquor , 
mull be very confiderable, as in folutions of bifmuth. Hence 
I in vain endeavoured, by can flic alkalies and lime-water, to 
deprive thefe folutions of this excels ; for when deprived even 
of only part of it, many ot the metals precipitated, and all 
II 2 would* 
