334 Prof. Bragg* and Mr. Kleeman on 



»!? 



four molecules of nitrogen and one of oxygen. The stopping- 

 power of nitrogen should be proportional to 2 v^14 or 7*48, 

 that of oxygen to 2^/16 or 8. Now (4 X 7*48 + 8) /5 = 7*58. 

 Therefore the stopping power of the CH 3 Br molecule as 

 compared to that of air should be 15 , 41/7*58 = 2*03 ; and the 

 range of the a. particle of Ra C should be 7/2*03 = 3*4 cm. 



A sufficient quantity of methyl bromide was prepared and 

 kept liquid in a freezing-mixture. The vessel containing our 

 apparatus was joined to an air-pump and exhausted till the 

 remaining pressure was equal to about 6 cm. of mercury. 

 The liquid was then allowed to pass in under atmospheric 

 pressure until the vessel was full of the gas. The pressure 

 was again reduced to t> cm., and vapour introduced until 

 the pressure in the vessel was once more equal to that of the 

 atmosphere. 



The ionization curve was then obtained. It is plotted in 

 fig. 2, and shows that the range is nearly as calculated from 

 the square-root law. 



It was not advisable to take it for granted that the vessel 

 contained vapour only ; and after a little time we hit upon a 

 method of determining this point satisfactorily. The method 

 also proved particularly useful in the case of mixtures of air 

 and vapours, as will be described presently. Immediately 

 after measuring the range of the particle in the gas, we 

 opened a communication between the vessel and an exhausted 

 globe of about one litre capacity. The capacity of the vessel 

 itself was 6 litres. The weight of the gas so drawn over was 

 compared with the weight of the air that came over under 

 similar conditions when the larger vessel contained air only; 

 and from this ratio the proportion of air and vapour could be 

 deduced. For example, the weight of the mixture of air and 

 methyl bromide was 2*599 grains. The corresponding weight 

 of air was *800 gram. (It was not, of course, at atmospheric 

 pressure.) Hence if a?= the ratio of the number of air 

 molecules to the number of molecules of CHBr, 



^ + 3-28 



2-599 



x + 1 



~ -800' 





== 3-25, 



X 



= 1/75. 



The extreme range of the a particle of Ra C in CH 3 Br is 

 shown by the curve to be 3*32, This is the distance from 

 the radium to the gauze of the ionization-chamber. The 



