VOL. LVI.] PHILOSOPHICAL TRANSACTIONS. 313 



made use of was 254 gniins, and was pretty exactly sufficient to vsaturate the 

 acid. The solution appeared, by pouring some of it into lime water, to contain 

 scarcely any fixed air. Therefore 254 grains of the volatile sal ammoniac 

 contain 134 grains of fixed air, i. e. .VoV of their weight. It appeared from 

 the same experiment, that l680 grains of the volatile salt saturate as much 

 acid as 1000 grains of marble. 



By another experiment, tried with some of the same parcel of volatile salt, it was 

 found to contain -tVoV of its weight of fixed air, and 1 643 grains of it saturated as 

 much acid as 1000 grains of marble. By a medium, the salt contained -r^VV of 

 its weight of fixed air; and l66l grains of it saturated as much acid as 1000 

 grains of marble. 



One thousand grains of marble were found to contain 407-J- grains of air, and 

 l66l grains of volatile sal ammoniac contain 885 grains. Therefore this 

 parcel of volatile sal ammoniac contains more fixed air, in proportion to the 

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

 885 to 4074-, or of 217 to 100. N. B. It is not unlikely, that the quantity of 

 fixed air may be found to differ considerably in different parcels of volatile sal 

 ammoniac; so that any one, who was to repeat these experiments, ought not 

 to be surprized if he was to find the result to differ considerably from that here 

 laid down. The same thing may be said of pearl ashes. 



Exper. 12. — ^This serves to account for a remarkable phenomenon, which I 

 formerly met with, on putting a solution of volatile sal ammoniac in water into 

 a solution of chalk in spirit of salt. The earth was precipitated, as might 

 naturally be expected: but what surprized me, was, that it was attended with a 

 considerable effervescence; though I was well assured, that the acid in the 

 solution of chalk, was perfectly neutralized. This is very easily accounted for 

 from the above-mentioned circumstance of volatile sal ammoniac containing 

 more fixed air in proportion to the quantity of acid that it can saturate, than 

 calcareous earths do. For the volatile alkali, by uniting to the acid, was 

 necessarily deprived of its fixed air. Part of this air united to the calcareous 

 earth, which was at the same time separated from the acid; but, as the earth 

 was not able to absorb the whole of the fixed air, the remainder flew off in an 

 elastic form, and so produced an effervescence. 



Exper. 13. — The same solution of volatile sal ammoniac made no precipitate, 

 when mixed with a solution of Epsom salt; though a mixture of it with a little 

 spirit of sal ammoniac, made with lime, immediately precipitated the magnesia 

 from the same solution of Epsom salt; as it ought to do according to Dr. Black's 

 account of the affinity of magnesia and volatile alkalies to acids. This experi- 

 ment is not so easily accounted for as the last; but I imagine, that the magnesia 

 is really separated from the acid by the volatile alkali; but that it is soluble in 



VOL. XII. s S s 



