Mr. Cavendishes Experiments on Aim 375 
The experiment was next repeated with fbrne impure tie-* 
phlogifticated air. The air wds very much diminifhed, but 
without the lead: cloud being produced in the lime-water. Nei- 
ther was any cloud produced when fixed air Was let up to it j 
but on the further addition of a little cauflic volatile alkali, a 
brown fediment was immediately perceived. 
Hence we may conclude, that the lime-water was faturated 
by fome acid formed during the operation ; as ill this cafe it is 
evident, that no earth could be precipitated by the fixed air 
alone, but that cauftic volatile alkali, on being added, would 
abforb the fixed air, and thus becoming mild, would imme- 
diately precipitate the earth ; whereas, if the earth in the lime- 
water had not been faturated with an acid, it would have been 
precipitated by the fixed air. As to the brown colour of the 
fediment, it mo ft likely proceeded from fome of the quickfilver 
having been difiolved. 
It mu ft be obferved, that if any fixed air, as well as acid, had 
been generated in thefe two experiments with the lime-water, 
a cloud muftTrave been at firft perceived in it, though that 
cloud would afterwards difappear by the earth being re-diftplved 
by the aeid ; for till the acid produced was fufficient to difi'olve 
the whole of the earth, fome of the remainder would be pre- 
cipitated by the fixed air $ fo that we may fafeiy conclude, that 
Ho fixed ait was generated in the operation. 
When the air is confined by foap-lees, the diminution pro- 
ceeds rather fafter than when it is confined by lime-water ; for 
which reafon, as Well as on account of their containing fo 
much more alkaline matter in proportion to their bulk, foap- 
lees feemed better adapted for experiments defigncd to investi- 
gate the nature of this acid, than lime-water. I accordingly 
made fome experiments to determine what degree of purity the 
Vol. LX XV. Ddd air 
