312 PHILOSOPHICAL TRANSACTIONS. [aNNO I766. 



pearl ashes would have absorbed the fixed air that passed through them. The 

 weight of the marble dissolved was 3] I4- grains. The loss of weight in effer- 

 vescence was 1254- grains. The whole empty space in the bottle and cylinder 

 was about 2700 grain measures: the excessive weight of that quantity of fixed 

 above an equal quantity of common air, is i-|- grains. Therefore the weight of 

 the fixed air discharged is 1274- grains. The cylinder with the filtering paper 

 was found to have increased 1^ grains in weight during the effervescence. The 

 empty space in the cylinder was about I160 grain measures: the excess of 

 weight of which quantity of fixed air, above an equal bulk of common air, is -f- 

 grains. Therefore the quantity of moisture condensed, in the filtering paper, is 

 one grain, or about -,4-5- part of the weight of the air discharged. 



As water has been already shown to absorb fixed air, it seemed not im- 

 probable but there might be some fixed air contained in the solution of marble 

 in spirit of salt; in which case the air discharged, during the effervescence, 

 could not be the whole of the fixed air in the marble. In order to see whether 

 this was the case, I poured some of the solution into lime water. It made 

 scarcely any precipitate; which, as the acid was entirely saturated with marble, 

 it would certainly have done if the solution had contained any fixed air. It 



appears therefore from this experiment, first, that marble contains — * = — - 



of its weight of fixed air; and 2dly, that the quantity of moisture, which flies 

 off" along with the fixed air in effervescence, is but trifling ; as I imagine that the 

 greatest part of what did fly off, must have been condensed in the filtering 

 paper. By another experiment, tried much in the same way, marble was found 

 to contain iVoV of its weight of fixed air. 



Exper. 1 1 . — Volatile sal ammoniac dissolves with too great rapidity in acids, 

 and makes too violent an eflfervescence, to allow one to try what quantity of 

 fixed air it contains in the foregoing manner: I therefore made use of the 

 following method. Three small phials were weighed together in the same scale. 

 The first contained some weak spirit of salt, the 2d contained some volatile sal 

 ammoniac in moderate sized lumps without powder, corked up to prevent 

 evaporation, and the 3d, intended for mixing the acid and alkali in, contained 

 only a little water, and was covered with a paper cap, to prevent the small jets 

 of liquor, which are thrown up during the effervescence, from escaping out of 

 the bottle. To prevent too violent an effervescence, the acid and alkali were 

 both added by a little at a time, care being taken that the acid should always 

 predominate in the mixture. Care was also taken always to cover the bottle 

 with the paper cap, as soon as any of the acid or alkali were added. As soon 

 as the mixture was finished, the 3 phials were weighed again; by which the loss 

 in effervescence was found to be 1 34 grains. The weight of the volatile salt 



