618 Dr. H. Geiger and Mr. E. Marsden on the Laws oj 



On account of the importance of these experiments further 

 measurements were made under somewhat different con- 

 ditions. The main difficulty in the previous experiments 

 arose from the fact, that owing to the rapid decay of the 

 source it was impossible to count in each case a sufficient 

 number of scintillations to obtain a true average value. In 

 the following set of measurements radium emanation in 

 equilibrium with its active deposit was used as source of 

 radiation. The source consisted of a conical glass tube 

 (fig. 6) of about 1| mm. internal diameter at its widest part, 

 the height of the cone being about 2'5 mm. 

 The end of the tube was closed airtight by Fig. 6. 



a sheet of mica of 0*62 cm. air equivalent. 

 This tube was filled with about 30 milli- 

 curies of highly purified emanation and 

 placed at R (fig. 3, p. 612) directly against 

 the mica window E, the air equivalent of 

 which was also 0*62 cm. 



The difficulty introduced by the employ- 

 ment of a particles of different velocities 

 (emanation, RaA, and Ka C) was elimi- 

 nated by using foils of approximately the 

 same air equivalent. The « particles there- 

 fore suffered the same reduction in velocity 

 in each foil, and the numbers of scattered 

 particles were therefore directly comparable. It was of 

 course impossible to obtain foils of exactly the same air 

 equivalent, but this difficulty was easily overcome by de- 

 termining the scattering for two foils of the same material, 

 one slightly smaller and the other slightly larger than a 

 standard thickness of 0*6 cm. air equivalent. 



Owing to the large variation with atomic weight of the 

 amount of scattering, the foils could not be all directly com- 

 pared with each other at the same angle. They were there- 

 fore compared in sets, the angle being chosen smaller for 

 the sets of lower atomic weight. Column VI. in the following- 

 table gives the mean results of the ratio of the number of 

 scattered particles to A 3/2 . 



The scattering of carbon was obtained by using thin 

 sheets of paraffin wax which contained about 85'2 per cent, 

 carbon and 14*8 per cent, hydrogen. The air equivalent of 

 the carbon was calculated from Bragg's law to be about 78 

 per cent, of the whole stopping power, and on account of the 

 low atomic weight of hydrogen all the scattering effect was 

 assumed due to the carbon. The measurements of the 

 scattering were made by comparison with that due to 

 aluminium foils of the same air equivalent. 



