89 



The absorption of a material used in a thin sheet natu- 

 rally appears greater than the absorption when the thickness 

 of material is increased, because the rays which are moving 

 obliquely are absorbed fivst. 



The absorption of y and X-rays appears to follow a purely 

 exponential law so far as experiment has been made. The o 

 rays are absorbed by molecules immediately on their produc- 

 tion, and cannot be said to show absorption effects. 



Having thus discussed the properties of the various rays 

 which do exist, it seems interesting to make an attempt at 

 the estimation of the properties of some rays which might 

 exist, though the fact has not been proved as yet. Radio- 

 active substances emit both positive and negative particles. 

 It does not seem at all out of place to consider the possi- 

 bility of the emission of neutral particles, such as, for ex- 

 ample, a pair consisting of one a or positive particle and 

 one S or negative jDarticle. The recent additions to our know- 

 ledge of the laws of absorption of a and /i particles give us 

 some grounds on which we may attempt to found an estimate 

 of the projDerties of such pairs. 



We know that the a particle moves in a rectilinear 

 course throughout its whole range, and passes tlirough the 

 atoms which it encounters without deflection. It does not 

 pursue a course which is straight on the whole, but zig-zag 

 in detail ; the direction and amount of a particle in motion 

 are the whole characteristics of that motion at any instant, 

 and no memory of any previous motion exists. If, there- 

 fore, a particle pursues a straight line in its motion as a 

 whole, it must keep to that line entirely and make no excur- 

 sions from side to side. We must, therefore, suppose that 

 an atom, or at least an a particle, endowed with sufficient 

 speed, can pass directly through another atom without appre- 

 ciable deflection. The a particle loses speed as it penetrates 



