v THE STUDY OF GASES 69 



means the inflammable air was made to pass through the 

 dry pearl-ashes ; whereby it must have been pretty effec- 

 tively deprived of any acid or watery vapours that could 

 have ascended along with it (Phil. Trans. 1766,^66, 153). 



The loss in weight was nf grains, but one grain of this 

 was due to the displacement of the common air in the 

 bottle by inflammable air ; the true weight of the inflam- 

 mable air was therefore lof grains. 



The weight of zinc used in this experiment was 254 grains. 

 A previous experiment (p. 66) had shown that one grain 

 of zinc gave 356 grain-measures of inflammable air. 

 The volume from 254 grains would therefore be 

 254x356 = 90424 grain-measures of gas. 



By combining the two experiments it was seen that 

 i of grains of the gas occupied the same volume as 

 90424 grains of water. The gas was therefore 8410 times 

 lighter than water, or roj times lighter than common air. 



Cavendish determines the weight of fixed air set 

 free by the action of acids on chalk and the alkalis, 

 The same method was applied to find the weight of fixed 

 air liberated by acids from various substances, blotting- 

 paper being substituted for the pearl-ashes because the 

 greater weight of gas demanded less careful drying. 

 Cavendish found that : 



1000 parts of marble lost 407 parts of fixed air. 



1000 parts of sal volatile lost 528 to 538 parts of fixed air. 



1000 parts of pearl-ashes lost 284 to 287 parts of fixed air. 



1000 parts of a crystalline salt obtained by saturating 

 pearl-ashes with fixed air lost 423 parts of fixed air. 



The quantity of acid required to liberate the fixed air 

 from each substance was also measured, and compared with 

 the weight of fixed air set free. It was found that the acid 

 which liberated 100 parts of fixed air from marble set free 

 109 parts from pearl-ashes, 217 from sal volatile, and 

 2 1 1 parts from the crystals prepared by the action of fixed 



