[ 1 53 3 
by all the chemical methods that are in ule j but 
could not find that water thus impregnated con- 
tained the leaft perceivable quantity of the acid. 
Mr. Hey, indeed, who affifted me in this exami- 
nation, found that diftilled water, impregnated with 
fixed air, did not mix fo readily with foap as the di- 
ftilled water itfelf ; but this was alfo the cafe when 
the fixed air had pafted through a long glals tube 
filled with alkaline falts, which, it may be fuppofed, 
would have imbibed any of the oil of vitriol that 
might have been contained in that air *. 
It is not improbable but that fixed air itfelf may 
be of the nature of an acid, though of a weak and 
peculiar fort. Mr. Bergman of Upfal, who honoured 
me with a letter upon the fubjedt, calls it the aerial 
acid, and, among other experiments to prove it to be 
an acid, he fays that it changes the blue juice of 
tournefole into red. 
The heat of boiling water will expell all the fixed 
air, if a phial containing the impregnated water be 
held in it; but it will often require above half an 
hour to do it completely. 
Dr. Percival, who is particularly attentive to every 
improvement in the medical art, and who has 
thought fo well of this impregnation as to prefcribe 
it in feveral cafes, informs me that it feems to be 
much ftronger, and fparkles more, like the true 
Pyrmont water, after it has been kept fome time. 
This circumftance, however, thews that, in time, the 
fixed air is more eafily difengaged from the water, and 
* An account of Mr. Hey’s experiments will be found in the 
Appendix to thefe papers. 
Vol. LXII. 
though. 
X 
