Various Gases hy the a Particles of Radium. 353 



line if the values of B for these substances were more in keep- 

 ing with those for C 6 H 6 and C 5 H ]2 . 



Since k is nearly the same for a number of gases, v/s is 

 also nearly the same. Thus the molecular volume is connected, 

 not very distantly, with the sum of the square roots of the 

 weights of the atoms which make up the molecule. 



Each of these physical properties which are so nearly 

 related is par fly additive, partly constitutive. For example, 

 the molecular volume of an organic molecule depends in part 

 on the sum of the volumes of the constituent atoms, and in 

 part on the mode of constitution. This suggests that there 

 is some fundamental and purely additive property of the 

 atom itself, on which various semi-additive properties are 

 based. For this reason it appears to be of great interest 

 that the stopping-power of the atom has shown itself to be 

 simply additive, so far as experiment has tried it; and at 

 the same time to be closely connected with the atomic volume, 

 the atomic refraction, and the rest. The additive nature of 

 the constant may be seen from the following table, in which 

 the observed stopping-powers of a number of gases are set 

 alongside those calculated from assumed values for H, C, 0, 

 and 01. 



Assume H 2 = *24, C 2 = *85, O 2 =r03, Cl 2 = 1*78, the air 

 molecule being taken as standard : — 



2 H 2 . C 2 H 4 . C 6 H G . C 5 H 12 . CH 4 0. C 2 H 6 0. 



Calculated 1-09 133 3*27 3-56 1-41 2-08 



Observed 1-11 1-35 1-33 3-59 1-43 2'00 



C 4 H 10 O. C0 2 . CC1 4 . CHC1 3 . C 2 H 5 C1. 



Calculated 3-41 1-47 3*98 321 2'34 



Observed 3*33 1-48 4'00 3'16 2-36 



It is of course too early to say that the stopping-power 

 has been proved to be a perfectly additive property of the 

 atom, yet it is clear enough that it is more so than any other 

 known property, except one. The more nearly experiment 

 shows it to be strictly additive, the greater will be its title 

 to rank with mass itself. I hope to begin soon a fresh and 

 more accurate set of experiments in the endeavour to find 

 to what extent the additive law holds. 



The near proportionality of the stopping-power to the 

 atomic square root is an effect which is quite apart from its 

 additive nature. Its existence is a connecting link between 

 the atomic weight on the one hand and the atomic volume, 

 refractive power, &c., on the other. The preliminary paper 



Phil. May. S. 6. Vol. 13, No. 75. March 1907. 2 



