Substance emitted from Thorium Compounds. 13 



other from Eimer & Amend of New York. The oxide is pre- 

 pared by the latter by igniting thorium nitrate obtained from 

 monazite sand. 



The amount of discharge caused by tli3 emulation is in- 

 creased several times by the conversion of the nitrate into 

 the oxide; but at the same time, the rate of discharge due 

 to the ordinary radiation emitted by the thorium is increased 

 in about an equal ratio. The conversion of the nitrate 

 into the oxide took place below a red heat. On heating 

 in a muffle for some time at white heat, the amount of 

 emanation continually diminished, till after four hours' ex- 

 posure to the heat, the rate of discharge due to the emanation 

 was only .^of the value immediately after its conversion into 

 oxide. 



Both thorium oxalate and sulphate act in a similar manner 

 to the nitrate ; but the emanation is still given off to a con- 

 siderable extent after continued heating. 



In considering the question of the origin and nature of the 

 emanation, two possible explanations naturally suggest them- 

 selves, viz. : — 



(1) That the emanation may be due to fine dust 

 particles of the radio-active substance emitted by the 

 thorium compounds. 



(2) That the emanation may be a vapour given off 

 from thorium compounds. 



The fact that the emanation can pass through metals and 

 large thicknesses of paper and through plugs of cotton-wool, 

 is strong evidence against the dust hypothesis. Special ex- 

 periments, however, were tried to settle the question. The 

 experiments of Aitken and Wilson * have shown that ordinary 

 air can be completely freed from dust particles by repeated 

 small expansions of the air over a water-surface. The dust 

 particles act as nuclei for the formation of small drops, and 

 are removed from the gas by the action of gravity. 



The experiment was repeated with thorium oxide present 

 in the vessel. The oxide was enclosed in a paper cylinder, 

 which allowed the emanation to pass through it. After 

 repeated expansions no cloud was formed, showing that for 

 the expansions used the particles of the emanation were too 

 small to become centres of condensation of the water-vapour. 

 We may therefore conclude, from this experiment, that the 

 emanation does not consist of dust particles of thorium oxide. 



It would be of interest to examine the behaviour of the 

 emanation for greater and more sudden expansions, after the 



* Trans. Rov. Soc. 1897. 



