14 Radio-active Substance emitted from Thorium Compounds. 



manner employed by 0. T. R. Wilson* in bis experiments on 

 tbe action of ions as centres of condensation. 



Tbe emanation may possibly be a vapour of thorium. There 

 is reason to believe that all metals and substances give off 

 vapour to some degree. If tbe radio-active power of thorium 

 is possessed by tbe molecules of the substance, it would be 

 expected that the vapour of the substance would be itself 

 radio-active for a short time, but the radio-active power 

 would diminish in consequence of the rapid radiation of 

 energy. Some information on this point could probably be 

 obtained by observation of the rate of diffusion of the ema- 

 nation into gases. It is hoped that experimental data of this 

 kind will lead to an approximate determination of the molecular 

 weight of the emanation. 



Experiments have been tried to see if the amount of the 

 emanation from thorium oxide is sufficient to appreciably 

 alter the pressure of the gas in an exhausted tube. The oxide 

 was placed in a bulb connected with a Pliicker spectroscopic 

 tube. The whole was exhausted, and the pressure noted by a 

 McLeod gauge. The bulb of thorium oxide was disconnected 

 from the main tube by means of a stopcock. The Pliicker 

 tube was refilled and exhausted again to the same pressure. 

 On connecting the two tubes together again, no appreciable 

 difference in the pressure or in the appearance of tbe discharge 

 from an induction-coil was observed. The spectrum of the 

 gas was unchanged. 



Experiments, which are still in progress, show that the 

 emanation possesses a very remarkable property. I have 

 found that the positive ion produced in a gas by the emanation 

 possesses the power of producing radio-activity in all sub- 

 stances on which it falls. This power of giving forth a radia- 

 tion lasts for several days. The radiation is of a more pene- 

 trating character than that given out by thorium or uranium. 

 The emanation from thorium compounds thus has properties 

 which the thorium itself does not possess. A more com- 

 plete account of the results obtained is reserved for a later 

 communication. 



McGill Universitv, Montreal, 

 September 13th, 1899. 



* Phil. Trans. Roy. Soc. vol. clxxxix. (1897;. 



