.'580 Dr. A. B. Wood on Long-range 



than the number through aluminium. Alternatively, if 

 all the long-range particles originate in the active source, 

 the number of scintillations observed through aluminium 

 will be exactly the same as the number through mica — 

 always assuming, of course, that the aluminium and mica 

 screens have the same air-equivalents. If, however, both 

 oxygen atoms and long-range a-particles are present, the 

 result will be a compromise between these two extremes, 

 i. e. the number through aluminium will be somewhat less 

 ihan the number through mica. An experiment of this 

 nature should therefore provide a crucial test of the origin 

 of the long-range particles. 



It has already been mentioned that the mica screen 

 consisted of two layers, each of air-equivalent 4*3 cm. — 

 total 8'6 cm., whilst the aluminium screen was made up of 

 four layers of 2'0 cm. air-equivalent and one layer of 05 cm. 

 air-equivalent — total 8'5 cm. It will then be seen that 

 -the "comparison" screens were, as nearly as practicable, of 

 equal stopping power, the aluminium screen, if at all, 

 having a slightly less air-equivalent than that of the mica. 

 In carrying out an experiment, a careful comparison was 

 "made of the number of scintillations observed when the 

 -8*6 cm. a-particles were absorbed in each of these screens, 

 the relative positions of the active source and zinc-sulphide 

 screen being fixed during an experiment whilst the absorb- 

 ing screens were interchanged. As a result of such 

 comparisons it was observed that the number of long-range 

 scintillations was almost the same in the two cases. A 

 slight excess, in favour of the mica, was however always 

 obtained, this excess amounting only to a small fraction — 

 less than 10 per cent, of the total. Thus it was found in 

 one comparative test that the number of scintillations 

 observed through the mica screen averaged 10*0 per 

 minute, whilst with the aluminium screen the average was 

 9*4 per minute. On another occasion the number through 

 the mica screen averaged 6'0 per minute, whilst aluminium 

 gave 5'5 per minute. The differences are small, and fall 

 very near to the limits of error of experiment. They are 

 significant, however, in suggesting, what otherwise seems 

 probable, that a small fraction, 7 or 8 per cent., of the 

 long-range particles comes from the mica, the greater 

 proportion, over 90 per cent., coming directly from the 

 active deposit. 



The two methods of analysis lead therefore to the same 

 result, viz. that the greater proportion of the long-range 



