Mr. Cavendish's Experiments on Air, 375 



The experiment was next repeated with fome impure de- 

 plilogiilicated air. The air was very much diminifhed, but 

 without the leall: cloud beiwg produced in the lime-water. Nei- 

 ther was any cloud produced when fixed air was let up to it ; 

 but on the further addition of a little cauflic volatile alkali, a 

 brown fediment >vas immediately perceived. 



Hence we may conclude, that the lime-water was faturated 

 by fome acid formed during the operation ; as in this cafe it is 

 evident, that no earth could be precipitated by the fixed air 

 alone, but that cauftic volatile alkali, on being added, wpuld 

 abforb the fixed air, and thus becoming miild, 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 mofl likely proceeded from fome of the quickfilver 

 having been diflblved. 



It muff: be obferved, that if any fixed air, as well as acid, had 

 been generated in thefe two experiments with the lime-water, 

 ■a cloud mull: have been at firft perceived in it, though that 

 cloud would afterwards difappear by the earth being re-diilolved 

 by the acid ; for till the acid produced was fufTicient to dilTolve 

 the whole of the earth, fome of the remainder would be pre- 

 cipitated by the fixed air ; fo that we may fafcly conclude, that , 

 no fixed air was generated in the operation. 



When the air is confined by foap-lees,. the diminution pro- 

 ceeds rather fifler than when it is confined by lime-v*-ater ; for 

 which realbuj as well as on account of their containing fo 

 much more alkaline matter in propmtion to their bulk, foap- 

 lees feemed better adapted for experiments defigned to invefti- 

 gate the nature of this acid, than lime-water. I accordingly 

 made fome experiments to determine what degree of purity the 

 Vol. LXXV. D d d ' air 



