(36 



DKNSITIES OF OXYUEN AND HYUR()(JEN 



The tube leadiiitij to tlie \m\n[) liail to be closed duriiiLC the iiitroiluetioii of the 

 hydidireii :uid the measuieineiit of its inessure in such a way that no leakage could 

 occur. A glass stopcock can often be lubricated so that when closed it will not 

 leak, but if opened and closed n-peatfilly it is liable to leak. In i'a<-l a stnpro<-k, in 

 all accurate work, ought to lie cousidcifd as an instrnnicnt lor irdiK-imj the tlow oi 

 a current of iras. 



Kic.. 23. — .Apparatus for receiving hydrogen and measuring its volume and pressure at constant tcmper.iturc. 



The counectiou lietween the measuring globes and the air [minp was therefore 

 made proof against leakage by using two stopcocks, and filling the space between 

 them with mercury at the atniospheiic pressure whenever it was desired to dose 

 tlie p!X.ssage. The two stopcocks are shown at e,f; a tid)e inserted at the lowest 

 iioint between them led lo the stopcock /■ and the funnel /, which was full of mer- 

 cury. If the stopcocks ^ and /' were open and the tube between them free from 

 mercury, a clear passage existed between the air-pump and the globes, ^^'hen a 

 sufficient vacuum had been obtained by the action of the piiniji, the stopcocks r and /' 

 were (dosed and /■ was opened. The pi'essure of the atmosphere then forced mer- 

 ciirv from the funnel to fill the space between c and /'," when /• was closed. It is 



