171. It has been shown in section 93 from experimental data that 1 gram of radium bromide at its minimum activity emits about 3·6 × 10^{10} α 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 and about equal to the minimum activity, the number of α particles projected per second by the emanation from 1 gram of radium bromide is about 3·6 × 10^{10}. It has been shown in section 152 that 463,000 times the amount of emanation produced per second is stored up in the radium. But, in a state of radio-active equilibrium, 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 α particle, it follows that the number of emanation particles present in 1 gram of radium bromide in radio-active equilibrium is 463,000 × 3·6 × 10^{10}, i.e. 1·7 × 10^{16}. Taking the number of hydrogen molecules in 1 c.c. of gas at atmospheric pressure and temperature as 3·6 × 10^{19} (section 39), the volume of the emanation from 1 gram of radium bromide is 4·6 × 10^{-4} cubic centimetres at atmospheric pressure and temperature. Assuming the composition of radium bromide as RaBr_{2}, the amount from 1 gram of radium in radio-active equilibrium is 0·82 cubic millimetres. Quite independently of any method of calculation it was early evident that the volume of the emanation was very small, for all the earlier attempts made to detect its presence by its volume were unsuccessful. It will be seen, however, that, when larger quantities of radium were available for experiment, the emanation has been collected in volume sufficiently large to measure.
In the case of thorium, the maximum quantity of emanation to be obtained from 1 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, stored in a non-emanating thorium compound, is only 87 times the rate of production, while in radium it is 463,000 times, and the rate of production of the emanation by radium is about 1 million
