[ RUTHERFORD | THE NEW GAS FROM RADIUM 28 
In the above table we see that the coefficient of interdiffusion follows 
the inverse order of the molecular weights. In cases of the simpler 
gases.it has been shown experimentally that the coefficient of inter- 
diffusion is approximately inversely proportional to the square root of 
the product of the molecular weight. If we apply these considerations 
to the emanations (K = -08 to -15) we see that it is a gas or a vapour 
of molecular weight (allowing a wide margin) probably lying between 
40 and 100. These numbers exclude the possibility of the substance 
being a vapour of radium, for it has already been shown by M. and 
Mme. Curie that the atomic weight of radium is greater than that of 
barium, We must therefore conclude that the emanation is in reality a 
heavy radioactive vapour or gas. 
On account of the rapid decay of the radiating power of thorium 
emanations, it is not possible to determine its coefficients of diffusion in 
the same way ; but special experiments show that it diffuses rapidly, and 
is also probably gaseous in character. The physical properties of these 
emanations or gases are most remarkable. The radium emanation not 
only continues for long intervals to be a source of radiation which is 
apparently similar in character to easily absorbed Rôntgen rays, but in 
some way manufactures from itself a positively charged substance, 
which travels to the negative electrode and becomes a source of secondary 
radioactivity. 
