578 Prof. E. Rutherford and Mr. F. Soddy on 



of giving an emanation, and of exciting radioactivity on 

 surrounding surfaces. We have confirmed and extended his 

 results by the following experiments. 



Two similar samples of thoria left sealed up for a week, 

 the one in a desiccated atmosphere, the other in air saturated 

 with water-vapour, showed an increase and decrease in 

 emanating power respectively. The moist sample possessed 

 nearly twice as much emanating power as the dry. More 

 complete desiccation, by sealing-up the specimens in vacuo 

 with phosphorus pentoxide for a month, did not further 

 reduce the emanating power. Some thoria mixed with 

 concentrated sulphuric acid gave about one half of the usual 

 amount of emanation when vigorously shaken. These 

 experiments show that the presence of water, although 

 producing a marked increase, is not essential for the 

 production of the phenomenon. 



Other experiments were made on the effect of light and 

 air on emanating power. The most useful result obtained is 

 that thoria does not change in emanating power when kept 

 in closed vessels under different conditions, but when exposed 

 to the air the emanating power varies within comparatively 

 narrow limits. 



Thorium Hydroxide. — This compound, like the oxide, has 

 its emanating power increased by water-vapour. A similar 

 experiment to that described for the oxide gave as the result 

 an emanating power of 400 per cent, of that of thoria for the 

 moist sample and 300 per cent, for the dry. Exposure to 

 the air for a short time again equalized the two values. 

 Carbon dioxide, which thorium hydroxide absorbs from the 

 air to the extent of 2 per cent, of its weight, is without 

 influence on the emanating power. 



Effects of Molecular Condition and State of Aggregation 

 of Ihorium on the Emanating JPoiuer. — Unlike the radio- 

 activity, the emanating power of thorium compounds is by 

 no means mainly controlled by the proportion of thorium 

 present. The effect of temperature in de-emanating thoria 

 and the high value of the emanating power of thorium 

 hydroxide illustrate this. Thorium sulphate, oxalate, and 

 nitrate possess but low emanating power, while thorium 

 carbonate has been obtained with a value five times as great 

 as that of* thoria. In general a dense crystalline compound 

 in not very fine powder possesses a much higher emanating 

 power than a light floury compound in a much finer state of 

 division. 



Solution, however, has been found generally to greatly 

 increase the emanating power of soluble thorium salts. In a 



