10 INAUGUKAL ADDRESS. 



we realise, as we do so, that this is not going very far, for we know 

 \ery little us yet of the way in which these materials are put together. 



So far, we have been considering the phenomena that att^end the 

 origination of the radiations. Let us now turn our attention to the 

 behaviour of the radiations during their passage through material 

 substances. The so-called absolution effects are most extraordinary, 

 and teach us further lessons of great interest. 



Let us, therefore, imagine ourselves able to project streams of 

 one or other of the new radiations through various substances, and to 

 watch the result. And, before going further, I had better explain 

 briefly how the watching is to be done. The fact is that as an alpha 

 or bet.a- ray passes through a gas it leaves behind a trail of electrons 

 loosened from the atoins which it has travei'sed, and that it is a com- 

 paratively simple matter to gather up these electrons, and so to follow 

 up the track of the ray. The loosened electrons are the delta rays, 

 and the mode of their unloosening is apparently just the same, no 

 matter to what agent it is due. I may repeat that the delta rays start 

 out from the parent atom A\'ith barely enough speed to enable them to 

 get clear away. Inasmuch as they are electrons in motion, they are 

 just the same as the beta rays of radium or the cathode rays in the 

 X-ray tube, but their speed is too sniall to give them the distinguish- 

 ing properties of these latter rays. 



When we consider the absorption effects we find that, in the first 

 place, there is the most remarkable rectilinear propagation of radia- 

 tions which are known to be material. A pencil of alpha or beta rays 

 projected in a certain direction can maintain that direction, in a 

 general sense, after having traversed many centimetres of a gas at 

 ordinaiy pressure. But a straight line 10 centimetres long, placed in 

 a gas, passes through something like a million molecules. If a stream 

 of paiiicles maintains its general direction after such a course, it 

 follows that the particles haA^e had no difficult passage through the 

 atoms which they have met. They cannot have gone round them, that 

 is to say, been ricochetted to and fro, and yet kept the main direction 

 in view ; that would only be possible if a guiding force acted all the 

 time, or if the particles were endowed with intelligence. We do indeed 

 find that a scattering of the particles occurs ; it has long been known 

 that beta particles can be turned out of the main stream and shot 

 into new directions, and Geiger, working at Manchester, has shown 

 recently that the alpha particles, although enormously more massive 

 than the beta, are also linl^le to be swung out of their course. But 

 such deviations cannot be likely to liappen frequently to a particle in 



