314 PHILOSOPHICAL TRANSACTIONS. [anNO 1766, 



water, when united to so great a proportion of fixed air, as is contained in a 

 portion of volatile sal ammoniac, sufficient to saturate the same quantity of acid. 

 The reason why the mixture of the solution of volatile sal ammoniac, with the 

 spirits of sal ammoniac made with lime, precipitates the magnesia from the 

 Epsom salt, is, that as the spirits made with lime contain no fixed air, the mix- 

 ture of these spirits with the solution of volatile sal ammoniac contains less air, 

 in proportion to the quantity of acid which it can saturate, than the solution of 

 volatile sal ammoniac by itself does. Volatile sal ammoniac requires a great 

 deal of water to dissolve it, and the solution has not near so strong a smell as 

 the spirits of sal ammoniac made with fixed alkali ; the reason of which is, that 

 the latter contain much less fixed air. But volatile sal ammoniac dissolves in 

 considerable quantity in weak spirits of sal ammoniac made with lime, and the 

 solution differs in no respect from the spirits made with fixed alkali. This is 

 a convenient way of procuring the mild spirits of sal ammoniac, as those made 

 with fixed alkali are seldom to be met with in the shops. 



Exper. 14. — ^The quantity of fixed air contained in pearl ashes was tried, by 

 mixing a solution of pearl ashes with diluted oil of vitrei, in the same manner 

 as was used for volatile sal ammoniac. As much of the solution was used as 

 contained 328^ grains of dry pearl ashes. The loss of effervescence was QO 

 grains. The mixture, which was perfectly neutralized, being then added to a 

 sufficient quantity of lime water, in order to see whether it contained any fixed air, 

 a precipitate was made, which being dried weighed S\ grains. Therefore, if 

 we suppose this precipitate to contain as much fixed air as an equal weight of 

 marble, which I am well assured cannot differ very considerably from the truth, 

 the fixed air in it is S-i- grains, and consequently the air in 328^- grains of the 

 pearl ashes, is QS-i- grains, i. e. tVoV of their weight. By another experiment 

 tried in the same way, they appeared to contain -jVjnr of their weight of fixed 

 air. 1558 grains of the pearl ashes were found to saturate as much acid as 

 1000 grains of marble. Therefore this parcel of pearl ashes contains more 

 air, in proportion to the quantity of acid that it can saturate, than marble does, 

 in the proportion of lOQ to 100. 



Exper. 15. — Dr. Black says that, by exposing a solution of salt of tartar for 

 a long time to the. open air, some crystals were formed in it, which seemed to 

 be nothing else but the vegetable alkali united to more than its usual proportion 

 of fixed air. This induced me to try whether I could not perform the same 

 thing more expeditiously, by furnishing the alkali with fixed air artificially; 

 which I did in the manner represented in fig. 18: where a represents a wide- 

 mouthed bottle, containing a solution of pearl ashes; Bb represents a round 

 wooden ring fastened over the mouth of the bottle, and secured with luting; 

 ,c is a bladder bound tight over the wooden ring. This bladder, being 



