68 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



trustworthy, because they depended so largely on an exact 

 knowledge of the density of the common air displaced by 

 the bladder, e.g., taking the density of common air as being 

 850 (instead of 800) times less than that of water, Cavendish 

 found inflammable air to be "9,200 times lighter than 

 water, or io T 8 o lighter than common air." 



Cavendish devises a new method of 

 weighing gases. In order to overcome 

 this difficulty Cavendish devised a method 

 of weighing directly the gas liberated by 

 the action of acids on a known weight of 

 metal or chalk. If the volume of the gas 

 had been determined in a separate ex- 

 periment the density could easily be calcu- 

 lated. Cavendish writes : 



" I endeavoured to find the weight of the 

 air discharged from a given quantity of 

 zinc by solution in the vitriolic acid in the 

 manner represented in Fig. 17. A is a 

 bottle filled near full with oil of vitriol 

 diluted with about six times its weight of 

 water : B is a glass-tube fitted into its 

 mouth and secured with lute i 1 C is a 

 glass cylinder fastened on the end of the 

 tube, and secured also with lute. The 

 cylinder has a small tube at its upper end 

 to let the inflammable air escape, and is 

 filled with dry pearl-ashes 2 in coarse powder. 

 The whole apparatus together with the zinc, which was. 

 intended to be put in, and the lute which was to be used 

 in securing the tube to the neck of the bottle, were first 

 weighed carefully; its weight was 11930 grains. The zinc 

 was then put in, and the tube put in its place. By this 



1 "The lute used for this purpose .... is composed of almond 

 powder, made into a paste with glue, and beat a good deal with a 

 heavy hammer. This is the strongest and most convenient lute I know 

 of" (loc. ?., p. 143, footnote). 



2 /.<?., potash. 



FIG. 17 CAVEN- 

 DISH'S APPAR- 

 ATUS FOR FIND- 

 ING THE WEIGHT 

 AND DENSITY OF 

 GASES. 



