160 RATE OF EMISSION OF ENERGY [CH. 
The small closed Dewar flask A contains the radium in a glass: 
tube &, immersed in the liquid to be employed. The flask A is 
surrounded by another Dewar bulb 5, containing the same liquid, 
so that no heat is communicated to A from the outside. The gas, 
liberated in the tube A, is collected in the usual way over water or 
mercury and its volume determined. By this method, measurements 
were made with liquid carbon dioxide, oxygen, and liquid hydrogen. 
Especial interest attaches to the results with liquid hydrogen. 
The rate of heat emission of the radium was found to be about 
the same in boiling carbon dioxide and oxygen, but Dewar? states 
that it was distinctly greater in liquid hydrogen. ‘This result, if 
confirmed, is of great interest, for 1t shows that while the rate of 
heat emission is practically unchanged between the range of tem- 
perature of liquid oxygen and carbon dioxide, the great relative 
drop in absolute temperature between liquid oxygen and hydrogen 
causes an increase in the heat emission. It will be shown in the 
next section that the heat emission of radium is directly connected 
with the radio-activity of that element. A change in the rate of 
heat emission must then involve a change in the radio-activity of 
radium. The conclusion that the heat emission of radium is greater 
in liquid hydrogen than at ordinary temperatures thus requires 
confirmation by direct measurements of the radio-activity. 
The use of liquid hydrogen is very convenient for demonstrat- 
ing the rate of heat emission from a small amount of radium. 
From 0:7 grams of radium bromide (which had been prepared only 
10 days previously) 73 ¢.c. of gas was given off per minute. 
In later experiments P. Curie (loc. cit.) found that the rate of 
emission of heat from a given quantity of radium depended upon 
the time which had elapsed since its preparation. The emission 
of heat was at first small, but after a month’s interval practically 
attamed a maximum. If a radium compound is dissolved and 
placed in a sealed tube, the rate of heat emission rises to the same 
maximum as that of an equal quantity of radium in the solid 
state. 
106. Connection of the heat emission with the radia- 
tions. The observations of Curie that the rate of heat emission 
1 Dewar, British Association, 1903. 
