on the fpecific Gravities , &c. of Saline Suhfances . 203 
tvhofe means it is extricated. But as this air is extricated from 
« 
metals, not only by acids, but alio by alkalies this luppofi- 
tion muft vanifh of courfe. 
The fame reafons militate with equal ftrength againft the 
fuppolition that an earth of any kind enters into the compofi- 
tion of this air ; nor is there an inftance of any earth rendered 
permanently fluid by any means, except in fparry air. Beiides, 
if it were a metallic earth, it muft necefiarily be fuppofed to 
be in a metallic ftate ; and how then could it efcape the aftion 
of all kind of acids ? for no acid is capable of decompounding 
inflammable air. Laftly, refpirable air cannot be faid to be the 
bails of inflammable air, unlefs we fuppofe that refpirable air 
enters into the compoiition of metals; for Dr. priestley has, 
by folar heat, extracted inflammable air from them in a veflel 
full of mercury, into which refpirable air had no accefs, and 
even in vacuo . Befides, refpirable air and phlogifton form 
other compounds very different from inflammable air, niz. 
fixed and phlogifticated airs as will prefently be feen. 
It may alfo be fairly urged againft all thefe fuppofltions, that 
they are not founded on any direft experiment, nor any known 
analogy, but merely gratuitous, or at leaft deduced from expe- 
riments inadequate to their fupport ; whereas the opinion that 
inflammable air is nothing elfe than phlogifton thrown into a 
fluid form by elementary fire, is direfUy founded on that expe- 
riment whereby inflammable air is feparated from metals by 
mere folar heat in the moft perfect vacuum, juft as fixed air 
united to marble and in a concrete ftate (in which it is nearly 
of equal denfity with gold) is feparated from the marble, and 
thrown into a permanently fluid form by heat alone. 
* Mem. ?ar. 1776, p. 687. 
D d 2 
III 
