VOL. LXXV.] PHILOSOPHICAL TRANSACTIONS. If 



tween short columns of a solution of litmus, the solution acquired a red colour, 

 and the air was diminished, conformably to what was observed by Dr. Priestley. 

 When lime-water was used, instead of the solution of litmus, and the spark was 

 continued till the air could be no further diminished, not the least cloud could 

 be perceived in the lime-water ; but the air was reduced to -§■ of its original bulk; 

 which is a greater diminution than it could have suffered by mere phlogistication, 

 as that is very little more than 4- of the whole. The experiment was next re- 

 peated with some impure dephlogisticated air. The air was very much diminished, 

 but without the least cloud being produced in the lime-water. Neither was any 

 cloud produced when fixed air was let up to it ; but on the further addition of a 

 little caustic volatile alkali, a brown sediment was immediately perceived. 



Hence we may conclude, that the lime-water was saturated by some acid formed 

 during the operation ; as in this case it is evident that no earth could be preci- 

 pitated by the fixed air alone, but that caustic volatile alkali, on being added, 

 would absorb the fixed air, and thus becoming mild, would immediately precipi- 

 tate the earth ; whereas, if the earth in the lime-water had not been saturated with 

 an acid, it would have been precipitated by the fixed air. As to the brown co- 

 lour of the sediment, it most likely proceeded from some of the quicksilver having 

 been dissolved. It must be observed, that if any fixed air, as well as acid, had 

 been generated in these two experiments with the lime-water, a cloud must have 

 been at first perceived in it, though that cloud would afterwards disappear by the 

 earth being re-dissolved by the acid ; for till the acid produced was sufficient to 

 dissolve the whole of the earth, some of the remainder would be precipitated by 

 the fixed air ; so that we may safely conclude, that no fixed air was generated in 

 the operation. 



When the air is confined by soap-lees, the diminution proceeds rather faster 

 than when it is confined by lime-water ; for which reason, as well as on account 

 of their containing so much more alkaline matter in proportion to their bulk, 

 soap-lees seemed better adapted for experiments designed to investigate the na- 

 ture of this acid, than lime-water. I accordingly made some experiments to de- 

 termine what degree of purity the air should be of, in order to be diminished 

 most readily, and to the greatest degree; and I found that when good dephlogisti- 

 cated air was used, the diminution was but small ; when perfectly phlogisticated 

 air was used, no sensible diminution took place ; but when 5 parts of pure de- 

 phlogisticated air were mixed with 3 parts of common air, almost the whole of 

 the air was made to disappear. It must be considered, that common air consists 

 of 1 part of dephlogisticated air, mixed with 4 of phlogisticated ; so that a mix- 

 ture of 5 parts of pure dephlogisticated air, and 3 of common air, is the same 

 thing as a mixture of 7 parts of dephlogisticated air with 3 of phlogisticated. 



VOL. XVI, D 



