[EUTHBHFOED-owBNs] THORIUM AND URANIUM RADIATION 11 



It was very early observed that the radiation from thorium oxide 

 was not constant, but varied in a most capricious manner. This was the 

 more peculiar as the sulphate and nitrate were fairly constant. All the 

 compounds of uranium also give out a radiation which remains remark- 

 ably constant and probably varies very little with time. Becquerel has 

 found that a specimen of uranium which had been kept in a dark room 

 for two years gave out the same intensity of radiation as at first. The 

 inconstancy of the radiation from thorium oxide was examined in 

 detail, as it was thought it might possibly give some clue as to the cause 

 and origin of the radiation emitted by these substances. 



It was found that if the substance was inclosed in a lead box with 

 a door, the rate of leak was much slower with the door open than closed. 

 The addition of a slight draught of air caused by opening or shutting 

 the door of the room diminished the rate of leak still more. Under 

 similar conditions the rate of leak due to the sulphate and nitrate of 

 thorium and the uranium compounds is not appreciably affected. The 

 sensitiveness of thorium oxide to slight currents of air is very remark- 

 able, and made it difficult to work with. With the air quite still, 

 the substance in a few minutes regained its normal activity. The 

 recovery was quite gradual. On covering the radio-active substance 

 with aluminum foil the action was reduced, but was still quite marked. 



In order to investigate the matter in more detail, the substance 

 was placed in an air-tight vessel. When a current of air was passed 

 through by means of a water pump on a pair of bellows, the rate of leak 

 rapidly diminished to about one-third of its value, and did not change 

 much with considerable increase in the velocity of the blast. On cutting 

 off the current of air, the substance gradually recovered its normal 

 state. The effect was independent of the amount of moisture present, 

 as air bubbled through water gave the same effect as air which had 

 passed through a column of calcium chloride and phosphorus pent- 

 oxide. It was independent of the presence of carbonic acid. A cur- 

 rent of coal gas gave the same effect as a current of air. 



The reduction of the rate of leak by a given current of air depends 

 on the thickness oi the layer of the active substance. With thin layers 

 the diminution was small, but with layers several millimetres thick the 

 action was greatly increased. Layers of thin aluminum foil over the 

 thorium oxide only partially diminished the effect. 



The effect persisted unchanged if the same air was passed back- 

 wards and forwards through the apparatus by means of a suitable 

 aspirator. This seems to show that it is not the presence of any sub- 

 stance in the air existing in small quantity, that produces the effect. A 

 large number of experiments of various kinds have been tried, but so 



