the a. Particles of Radium. 335 



latter was 2 mm. deep, and half this, viz. 1 mm., should be 

 added in order to get the true range, which may therefore 

 be taken as 3 "42. The corresponding range in air is 7*06. 



Hence, if x — stopping power of the methyl-bromide 

 molecule, 



l + 7 5.r _ 7M36 _ 

 76 -3-42 



.-. x = 2-09, 



which is very nearly the value already calculated on the square- 

 root law. The errors of experiment were not so small that this 

 almost absolute agreement can be looked on as more than 

 accidental. It will be seen, however, that all the gases ex- 

 amined gave good results. 



The curve of a gas is different in some important respects 

 from that which is obtained in examining the loss of range 

 in metal films. In the latter case, all the ordinates are re- 

 duced from those of the normal air-curve by nearly the same 

 amount ; in the former, they are reduced in nearly the same 

 proportion. Again, in the case of the metal film (provided 

 it is uniform) the abscissas are unaltered ; in "the case of the 

 gas, the abscissae are all altered in the same proportion and 

 show the amount of ionization produced by the a particle in 

 traversing the gas. For example, the extreme abscissa of 

 the Ra C group has the value 666 in the gas and 547 in air ; 

 thus the ionization per cm. is 666/547 or 1*27 times greater 

 in the gas than in the air. Since, however, the range is 

 2*07 times shorter, the ratio of the total ionization in the gas 

 to the total ionization in air is only l , 27/2"07 = *62 nearly. 



In most of the complex gases examined the a. particle 

 produces less ionization than in air. We propose to make a 

 further examination of this point. 



In the case of the other gases, the procedure was very 

 similar. Carbon tetrachloride, methyl iodide, and ether are 

 liquid at ordinary temperatures and pressures, and in their 

 case we introduced the necessary quantity of liquid to give a 

 vapour pressure of about two-thirds that of saturation, and 

 then filled up with air. We found it better to do this than 

 to attempt the experiment with low pressures in our vessel, 

 for the latter had so many joints and stuffing-boxes that it 

 would not keep a vacuum very well. It was, however, quite 

 airtight enough to retain a mixture unchanged for hours 

 provided it was at atmospheric pressure. The proportion of 

 air to vapour was afterwards determined in the manner already 

 described. 



2 A 2 



