1437 
continued without our being certain of the temperature. The height of 
the surface of the liquid gas can be read by means of a float. 
This consists of a new-silver box p,, suspended from a weak 
spring p,, which spring is attached to a rod p,. This rod is 
movable in a packing tube, which is fastened to the upper end of 
a glass tube qg, carried by the lid A. Beside the spring and also 
hanging from the rod, is a flat rod which is provided with a scale 
at its lower end. [n consideration of the very small density of the 
liquid hydrogen (*/,,), the float is made very light. When the float 
reaches the surface of the liquid by the moving down of the spring, 
this is indicated by the shortening of the spring, and the height of 
the liquid can be read on the scale and on the rod. 
Before pouring in the liquid gas, the float is regulated to the 
height to which the vessel is to be filled. Before beginning the 
measurements, the spring is pressed down as far as is necessary 
to make the lengthening of it show when the liquid is so far eva- 
porated that the measurements must be stopped. 
The evaporated hydrogen is carried off by L,. The tube R, the 
extremity of which is placed in mercury, serves as a safety. 
In order to be certain of the insulation of the vacuum glass, and to 
avoid currents which might be injurious to the constancy of the tension 
of the battery, a piece of amber is interposed in the tube Z,. To 
prevent the amber from being cooled too much by the filling, the cold 
vapours are carried off by a supplementary tube L,, which is coupled 
off as scon as the filling is completed. When the evaporation of the 
bath has become stationary, a curreut of air a little warmer than 
that of the room directed upon the amber is sufficient to maintain 
the insulation. This current cf air is given by a reservoir of com- 
pressed air, the air flows through a long tube, part of which is 
warmed by hot water. 
The connection of the piece of amber, g, which is sealed to the vacuum 
glass, with the tube D, of the principal ionisation chamber, is very 
carefully made, to insure an airtight closing, and thereby to prevent 
the possibility of moisture penetrating to the space between the tube 
and the vacuum glass. The currents of cold air that come down are 
kept away by a paper screen. The water that runs down the glass 
from the lid must also be disposed of. The very low temperature 
of the vapours inside the lid causes frost to settle on it during the 
filling, which thaws afterwards. After the filling is finished, the 
condensation of water vapour out of the air continues; the water 
thus formed, is absorbed by cotton wool above the paper screen 
we mentioned, and below it by filter-paper. A current of dry slightly 
