276 Mr. W. Sutherland on 



absorption for a large number of gases appeared to be pro- 

 portional to the density within the limits of experimental 

 error, the coefficient for hydrogen was exceptional to an 

 extent decidedly beyond possible experimental error. In his 

 experiments, Lenard showed that if J is the intensity of a 

 Lenard stream at its source, J that at a distance r from the 

 source in a substance whose coefficient of absorption is A, 



J=J «- A 7^, 



and determined A for various gases at a pressure of one atmo. 

 As the densities of these gases are as their molecular weights, 

 with that of hydrogen = 2, he shows the relation of A to the 

 density of different gases by tabulating values of A/m ; while 

 according to our reasoning RA/m would be expected to 

 be constant. The following table contains Lenard's values 

 of 10 3 A/m, and relative values of R as given in my paper on 

 the " Attraction of Unlike Molecules — The Diffusion of Gases," 

 Phil. Mag. [5] xxxviii., being half the cube-root of the 

 limiting space occupied by a gramme-molecule of the sub- 

 stance and controlled by comparison with molecular dimen- 

 sions as given by experiments on the viscosity of gases ; the 

 last row contains the product 10 3 RA/m : — 



H 2 . CH 4 . CO. C 2 H 4 . N 2 . 2 . C0 2 . N 2 0. SO a . 



10 3 A/m ... 237 124 122 132 113 126 115 102 133 



R , 1025 147 1-35 1-75 1415 134 156 1-535 1-63 



10 3 RA/w... 243 182 165 231 160 169 179 157 217 



Thus while Lenard's approximate constant ranges from 102 

 to 237, the one to which we have been led ranges from 157 to 

 243, which is an improvement. The really striking point 

 about Lenard's discovery, however, is that when A is divided 

 by density, the range in value is from 2070 for paper to 5610 

 for hydrogen at one atmo ; the results for many substances 

 such as gold and hydrogen at 1/228 atmo falling between 

 these extremes. The fact that the value of Afp for a rare 

 gas is almost the same as for a dense solid, would seem to 

 indicate that it is only when an electron strikes an atom 

 almost in the direction of a normal that the most important 

 part of the absorption of energy occurs; for if this is so, 

 the chance of an electron's encountering an atom in a solid 

 normally, while threading its way through the interstices, 

 being the same as if it could pass through all the atoms which 

 it does not meet normally, the absorption of energy from an 

 electron by a number of atoms should be the same whether 

 they are as close as in a solid or as wide apart as in a rarefied 

 gas. Thus probably the coefficient of absorption for a solid 



