466 Emission of Negative Electricity by Thorium. 



of thorium, so as to increase the effect ; and the side-tube, 

 after haying been kept some time in liquid air, was sealed off, 

 in order to avoid the irregularities which it had been found 

 to introduce. Immediately before removing the side-tube, 

 the positive leak was 17*2 divisions per hour, the negative. 6'0. 

 The liquid-air tube was then sealed off, and the rate of leak 

 at once measured. Starting the observations four minutes 

 after removing the tube, the successive results obtained were — 

 positive leak, 88 '5 per hour ; negative leak, 33*4 per hour ; 

 positive leak, 90 per hour. This large and rapid rise shows 

 clearly that a great part of the effect observed must be directly 

 due to the emanation, which begins to accumulate in the bulb 

 as soon as the cooled side-tube is removed. Owing to its 

 rapid rate of decay (half value in about 1 minute), it only 

 requires a few minutes for the maximum amount to collect. 



The bulb containing the thorium was next sealed off, and 

 the positive leak from 6 minutes to 16 minutes afterwards 

 was found to be at the rate of 25*8 divisions per hour. In 

 this space of time the amount of emanation in the testing- 

 vessel would have already become almost negligible, and this 

 residual leak was no doubt due to rays given off by the 

 excited activity on the walls. It fell off slowly, at a rate 

 which was approximately that of thorium-excited activity. 



From the above experiments, it is evident that in the case 

 of thorium as well as that of radium the emanation gives off 

 a quantity of slow negative rays in changing to the excited 

 activity. In air at the ordinary pressure these rays would be 

 rapidly absorbed ; but at the pressures used in these experi- 

 ments they reach the electroscope and discharge it. The 

 magnitude of the positive leak obtained (compared with the 

 negative) makes it extremely probable that the amount of 

 negative electricity given out by the emanation is considerably 

 greater than the positive electricity emitted in the form of 

 a, rays. If this is the case, the residue from which the excited 

 activity is derived must be formed, with a positive charge, 

 and its appearance on the negative electrode in an electric 

 field is explained. 



In conclusion I have only to express my best thanks to 

 Prof. J. J. Thomson, to whose kind interest and advice during 

 the course of the above experiments, which were carried out 

 at the Cavendish Laboratory, I am greatly indebted. 



