246 RADIO-ACTIVE EMANATIONS [CH. 
161. It was found! that the activity of the thorium emanation 
decayed at the same rate, when condensed in the spiral at the 
temperature of liquid air, as at ordinary temperatures. This is in 
accord with results of a similar kind obtained by P. Curie for the 
radium emanation (section 136), and shows that the value of the 
radio-active constant is unaffected by wide variations of tempera- 
ture. 
Amount of Emanation from Radium and Thoriwm. 
162. It has been shown in section 104, that 1 gram of radium 
emits about 10" a@ particles per second. Since the activity due to 
the emanation stored up in radium, when in a state of radio-active 
equilibrium, is about one quarter of the whole, the number of a 
particles projected per second by the emanation from 1 gram of 
radium is about 2°5 x10! It has been shown in section 143, 
that 463,000 times the amount of emanation produced per second 
is stored up in the radium. But in a state of radio-active equi- 
librium, the number of emanation particles breaking up per second 
is equal to the number produced per second. Assuming that each 
emanation particle in breaking up expels one « particle, it follows 
that the number of emanation particles, present in 1 gram of 
radium in radio-active equilibrium, is 463,000 x 2°5 x 10”, ze. 
1:2 x 10%. Taking the number of hydrogen molecules in 1 cc. of gas 
at atmospheric pressure and temperature as 3°6 x 10” (section 39), 
the volume of the emanation from 1 gram of radium is 3°3 x 107 | 
cubic centimetre at atmospheric pressure and temperature. Quite 
independently of any method of calculation, it is evident that the 
volume of the emanation is very small, for attempts made to 
detect its presence by its volume have so far failed. It is probable, 
however, from the above calculation, that, when larger quantities 
of radium are available for experiment, the emanation will be 
collected in volume sufficiently large to measure. 
In the case of thorium, the maximum quantity of emanation to 
be obtained from | gram of the solid is very minute, both on account 
of the small activity of thorium and of the rapid break up of the 
emanation after its production. Since the amount of emanation, 
1 Rutherford and Soddy, Phil. Mag. May, 1903. 
