4 Prof. E. Rutherford on a Radio-active 



greatly diaiinished. The amount of diminution is to a great 

 extent independent of the electromotive force acting between 

 the plates. Under similar conditions with uranium, the rate 

 of leak is not appreciably affected. With a thin layer of 

 oxide, the diminution of the rate of leak is small ; but with a 

 thick layer of oxide, the rate of leak may be reduced to less 

 than one-third of its previous value. If two thicknesses of 

 foolscap-paper are placed over the thorium oxide, the resulting 

 rate of leak between the plates may be diminished to less 

 than 2V °f i* s va l ue by a slight continuous blast of air from 

 a gasometer or bellows. 



The phenomena exhibited by thorium compounds receive a 

 complete explanation if we suppose that, in addition to the 

 ordinary radiation, a large number of radio-active particles 

 are given out from the mass of the active substance. This 

 " emanation " can pass through considerable thicknesses of 

 paper. The radio-active particles emitted by the thorium 

 compounds gradually diffuse through the gas in its neighbour- 

 hood and become centres of ionization throughout the gas. 

 The fact that the effect of air-currents is only observed to a 

 slight extent with thin layers of thorium oxide is due to the 

 preponderance, in that case, of the rate of leak due to the ordi- 

 nary radiation over that due to the emanation. With a thick 

 layer of thorium oxide, ike rate of leak due to the ordinary 

 radiation is practically that due to a thin surface-layer, as the 

 radiation can only penetrate a short distance through the 

 salt. On the other hand, the " emanation " is able to diffuse 

 from a distance of several millimetres below the surface of the 

 compound, and the rate of leak due to it becomes much greater 

 than that due to the radiation alone. 



The explanation of the action of slight currents of air is 

 clear on the " emanation " theory. Since the radio-active 

 particles are not affected by an electrical field, extremely 

 minute motions of air, if continuous, remove many of the 

 radio-active centres from between the plates. It will be 

 shown shortly that the emanation continues to ionize the gas 

 in its neighbourhood for several minutes, so that the removal 

 of the particles from between the plates diminishes the rate of 

 discharge between the plates. 



Duration of the Radio-activity of the Emanation. 



The emanation gradually loses its radio-active power. The 

 following method was adopted to determine the rate of decay 

 of the intensity of the radiation of the radio-active particles 

 emitted by thorium oxide. 



