12 Prof. E. Rutherford on a Radio-active [. 



or 



i =e -Kt. 

 i ' 



i. e., the current through the gas diminishes in a geometrical 

 progression. After 20 minutes the current through the gas 

 is only about one millionth part of its initial value. 



It has been shown that e~ kt =\ when £ — 60 seconds. 



Therefore X = g 1 ^ , 



and AT q 



or the total number of ions produced per second when a steady 

 state is reached is 86 times the number of ions supplied per 

 second by the emanation. 



The amount of emanation from thorium oxide increases with 

 the thickness of the layer. When 1 gramme of thorium oxide 

 was spread over a surface of 25 ems., the amount of discharge 

 due to the ordinary radiation had practically reached a 

 maximum. The rate of leak due to the emanation for the 

 same thickness was small. With 9 grammes of oxide spread 

 over the same area, the rate of leak due to the emanation had 

 reached about half its maximum value, which for that case 

 corresponded to four times the rate of leak caused by the 

 ordinary radiation. The emanation thus still preserves its 

 radio-active properties after diffusing through several milli- 

 metres of thorium compound. 



The emanation is given out whatever the gas by which 

 the thorium is surrounded. The action is very similar 

 whether air, oxygen, hydrogen, or carbonic acid is used. 



The rate of discharge due to the emanation diminishes 

 with lowering of the pressure of the air surrounding it. Only 

 a few observations have been made, but the results seem to 

 point to a uniform rate of emission of the emanation at all 

 pressures ; but since the intensity of the ionization of the gas 

 varies directly as the pressure, the rate of leak decreases with 

 lowering of the pressure. 



The amount of the emanation, so far as the experiments 

 have gone, is also independent of the quantity of water-vapour 

 present. 



The power of emitting radio-active particles is not pos- 

 sessed to any appreciable extent by other radio-active sub- 

 stances besides thorium. All the compounds of thorium 

 examined possess it to a marked degree, and it is especially 

 large in the oxide. Two different specimens of the oxide have 

 been used, one obtained from Schuchart of Germany, and the 



