the a. Particles of Radium. 333 



Since the atomic weight is of such consideration in this 

 law, it is more direct to state the results in another way. 

 Instead of comparing the stopping powers of strata of metal 

 and air of equal weights, we can compare the stopping powers 

 of strata containing equal numbers of atoms, and therefore 

 the stopping powers of individual atoms. For example, a 

 stratum of silver produces the same effect as a stratum of 

 air whose weight is 2*41 times greater. Thus for equal 

 weights silver stops 1/2'4I times as much as air ; but for 

 equal numbers of atoms it stops 108/14*4 x 2*41 times as much 

 as air. This ratio, which is equal to 3" 11, may be called the 

 " stopping power " of the silver atom, referred to the air 

 atom as a standard. The latter is an imaginary atom having 

 an atomic weight 14'4, a molecular weight 28*8, and an 

 atomic square root 3' 79. 



The stopping powers of the various metals examined will 

 be set out below, in conjunction with those of certain £ases. 



When we found that a simple law seemed to cover the 

 behaviour of substances differing so widely in all their pro- 

 perties as those enumerated, we thought it advisable to 

 examine such other substances as were available. We were- 

 unable to obtain other metal films; and we therefore turned 

 our attention to gases. Now no striking evidence for the 

 square-root law could be obtained from an examination of 

 gases whose atomic weights were nearly the same as that of 

 air, such as oxygen or nitrogen. Indeed, it had already been 

 shown by Strutt that their behaviour was very much the same 

 as that of air. Nevertheless, it was not to be forgotten that 

 their proved properties were not against the square-root law. 



We therefore made experiments with the following gases, 

 which contained atoms whose weights were very different 

 from those of air atoms: — 



Methyl bromide, methyl iodide, ethyl chloride, carbon 

 tetrachloride, ether, and hydrogen. 



Methyl bromide was a most suitable gas for our purpose. 

 The ratio of its molecular weight to that of air is 3*28. If 5 

 then, the loss of range in passing through this gas were 

 proportional to the density, the range of the particle from 

 Ha C would be only about 7/3'28 or 2*3 cm., provided the 

 gas were at atmospheric pressure. But if the square- root 

 law were true, the range would be much greater, and could 

 be calculated thus : — 



The carbon atom should contribute a stopping power pro- 

 portional to \/12 7 the three hydrogen atoms to 3x/l, and 

 the bromine atom to v'80. Total 3'46 +3-4- 8'95 = 15*41. 

 In air the stopping power should be the average of that of 



Phil. Mag. S. 6. Vol. 10. No. 57. Sept. 1905. 2 A 



