574 Prof. Rutherford and Mr. Soddy en 



sharply denned. It is rather a matter of remark that con- 

 densation can occur at all with such sparse distribution of 

 emanation particles in the gas, for in order for condensation 

 to take place the particles must first approach within each 

 other's sphere of influence. 



]^ow consider the case of the radium emanation. The rate 

 of decay is about 5000 times slower than that of the thorium 

 emanation, and consequently the actual number that must 

 be present to produce the same number of rays per second in 

 the two cases must be of the order of 5000 times greater 

 for the radium than for the thorium emanation. This con- 

 clusion involves only the assumptions that the same number 

 of rays are produced by a particle of emanation in each case, 

 and that the rays expelled produce in the passage through 

 the gas the same number of ions. The number of particles 

 that must be present for the electrometer to detect them in 

 this experiment must therefore be about 5000 x 1500, i. e. of 

 the order of 10". The difference of behayiour in the two 

 cases is well explained in the view that for equal electrical 

 vfeds the number of radium emanation particles must be far 

 larger than the number of thorium emanation particles. It 

 is to be expected that the probability of the particles coming 

 into each other's sphere of influence will increase very rapidly 

 as the concentration of the particles increases, and that in 

 the case of the radium emanation, once the temperature of 

 condensation is attained all but a negligibly small proportion 

 of the total number of particles present will condense in a 

 very short time. In the case of the thorium emanation, 

 however, the temperature might be far below that of con- 

 densation, and yet a considerable proportion remain uncon- 

 densed for comparatively long intervals. On this view the 

 experimental results obtained are exactly accounted for. 

 A greater proportion condenses the longer the time allowed 

 for condensation under the same conditions. The con- 

 densation occurs more rapidly in hydrogen than in oxygen, 

 owing to the diffusion being greater in the former gas. 

 For the same reason the condensation occurs faster the 

 lower the pressure of the gas present. Finally, when the 

 emanation is carried by a steady gas-stream, a less proportion 

 condenses than in the other cases, because the concentration 

 of emanation particles per cubic centimetre of gas is less 

 under these conditions. 



Decay of Activity of the Condensed Emanation. 

 Some experiments were made to test whether the rate of 

 decay of activity of the thorium emanation was altered at the 



