540 Sir E. Rutherford on Collision of 



scintillations outside the range of the a particles. It seems 

 probable that the large number of scintillations observed by 

 Marsden (loc. cit.) from a nickel plate coated with radium C 

 were mainly due, not to H atoms, but to high-velocity N and 

 atoms produced from the air between the source and the 

 screen. 



§ 3. Source of radiation. 



While the use of a-ray tubes as an intense source of 

 radiation has many advantages, it has the drawback that 

 the a, radiation is heterogeneous arising from the three 

 products radium A, radium C, and the emanation. In 

 addition, it is difficult to make a-ray tubes of uniform 

 thickness whose stopping power is less than two centimetres 

 of air. For these reasons, I have discarded the use of a-ray 

 tubes and have conducted the majority of the experiments 

 with a homogeneous source of radiation, consisting of the 

 active deposit of radium. Twenty minutes after removal 

 from the emanation, the a radiation arises entirely from 

 radium C and is homogeneous with a range in air of 7 cm. 

 A brief account will now be given of the method for 

 obtaining an intense source of radiation of convenient 

 dimensions. The source usually consisted of a circular 

 bevelled brass disk which was screwed on the lower end of 

 a glass stopcock. The emanation, after removal from the 

 radium solution, was sparked with oxygen to remove excess 

 of hydrogen until the volume was reduced to about 0'5 c.c. 

 This emanation was introduced by means of a mercury trough 

 into a small transfer pump and the mercury r;iised until its 

 level was 1 or 2 mm. below the disk to be activated. The 

 disk was connected through the stopcock with the negative 

 pole of the lighting circuit and the mercury with the positive 

 pole, in order to concentrate to some extent' the active matter 

 on the surface of the disk. After two hours' exposure, the 

 emanation was pumped out and the active disk removed. 

 Theoretically, in order to obtain the maximum activity, the 

 exposure to the emanation should be more than three hours, 

 but in practice it is found that an exposure of two hours gives 

 more activity, while an exposure of twenty-four hours gives 

 much less than an exposure of two hours. This anomalous 

 effect had been previously observed by Ratner *, and is 

 apparently due to the loss of active matter from the disk 

 through the intermediary of the electric wind. 



Using a large quantity of emanation, it is possible to 



* Katner, Phil. Mag. xxxiv. p. 429 (1917) ; xxxvi. p. 397 (1918). 



