(915 ) 
from whieh it cannot be removed by mereury. Of course the cock 
must be used in a vertical position, i.e. with the pivot / turned 
upwards and the cavity / turned downwards as indicated in the 
figure. The cock being tilled with mercury before use, the cavity 
F is filled with mercury up to the horizontal tube, and the 
passing gas is thus entirely shut off from the inevitable packing 
material at G. The handling of these cocks is greatly facilitated by 
the construction of the pivot /. Instead of consisting in a single 
cone, as it generally does, it consists in a broader cone, which 
terminates at the top in a very steep, truncated cone, which is 
almost as wide as the bore of the cock. In this way we achieve 
that when turning the handle, the opening only very slowly increases, 
which renders a very fine regulation possible. Particularly when 
first mercury is to pass through the cock, and then gas which has 
much less friction (this takes place in different manipulations see 
below, p. 916) it is of great importance that the width of the 
opening can be very accurately regulated. 
