56 
pressure or relieved therefrom. In the former case it must 
be alternately raised and depressed from 30 inches below b 
up to that level. In the latter it must be raised and 
depressed from the level of b to 30 inches above it. Castor 
oil is a useful medium to prevent the passage of air between 
mercury and the glass vessels. 
It is important to add a little sulphuric acid to the mer- 
cury, in order to remove the film of water which adheres to 
the inside of the globe b. On this account it would, perhaps, 
be desirable to substitute a plug of glass for the indiarubber 
one between a and b . 
