Artificial Disintegration of Light Elements. 817 



curves A, B, C, D represent the effects due to a particles o£ 

 initial ranges 8'6, I'O, 6*0, and 4*9 cm. respectively. The 

 ranges of! the particles are more than twice as great as for 

 those liberated from nitrogen. It is seen that the variation 

 of the number with velocity is very rapid. There is a 

 marked difference between -curves B and C, and it is doubtful 

 whether the rays of range 4*9 cm. give any particles at all, 

 for the numbers include the effect due to the "natural" 

 scintillations from the source. 



It was found exceedingly difficult to fix the end of the 

 range of the particles with accuracy on account of: the small- 

 ness of the numbers of scintillations. To increase the effect, 

 a stronger magnetic field, was used to bend away the /3 rays 

 and the source brought as close to the screen as the lumi- 

 nosity due to 7 rays and residual ff rays allowed. The results 

 as a whole showed that the ranges of the particles were 

 approximately proportional to the ranges of the incident 

 a rays, but a special series of experiments will be required 

 to fix the ranges with the accuracy desired. It is an impor- 

 tant question to decide whether such a proportionality 

 exists, as it may help to throw light on the mechanism 

 involved in the liberation of these swift particles. It seems 

 certain, however, that some of the particles from aluminium 

 due to u rays of range 7 cm. have a range of at least 90 cm. 

 of air. The number of scintillations was too small to follow- 

 the absorption further. As it is, the experiments have been 

 carried out to the point of detecting 1 particle per minute 

 for 20 mgs. at the standard distance of 3'5 cm. 



Direction of escape of Particles. 



From analogy with the projection of H atoms by a par- 

 ticles from hydrogen, it is to be expected that the great 

 majority of the particles liberated from the elements would 

 be expelled in the direction of the a particles. In the case 

 of aluminium, however, it was found that the direction of 

 escape of the particles was to a large extent independent of 

 the direction of the impinging a particles. Nearly as many 

 were expelled in the backward as in the forward direction. 

 This effect was first observed when an aluminium instead of a 

 brass disk was.coated with active deposit, and used as a source 

 of a ra} T s in an atmosphere of oxygen. The aluminium plate 

 in front of the source was removed. The particles which fall 

 on the zinc-sulphide screen can be liberated only by y. rays, 

 which are fired into # the aluminium disk in a direction away 

 from the zinc screen. 



This type of experiment is, however, open to the objection 



