the Corpuscular Hypothesis of the y and X Rays, 397 



There are therefore three main subjects of measurement in 

 respect to each entity: (a) the expenditure of energy along 

 the path, (b) the form of the deflexion oval, (c) the chance 

 of conversion of form. Let us consider to what extent these 

 measurements have ,been made, and also some methods of 

 making them. 



Let us take the a particle first. The case is an especially 

 simple one because there is no conversion of form, and very 

 little chance of deflexion until the speed has greatly diminished 

 and the range is nearly completed. Hence the particle's 

 properties are almost entirely expressed when its range is 

 determined; and this has been done with some thoroughness. 

 The feeble but very interesting deflexions which do take place 

 have been measured by Greiger. Our knowledge of the 

 a particle is fairly complete in the sense that we know what 

 to expect when any given screen is placed in the path of any 

 given stream of radiation. We may go on to consider some 

 of the other radiations of which we know less. 



The X and 7 rays have also their special points of simplicity, 

 but they form an almost exact antithesis to the a rays. Here 

 it appears that the expenditure of energy along the track is 

 either negligible or non-existent. The rays do not ionize 

 directly. Nor is the deflexion oval a very important thing. 

 The most important feature is the chance of transmutation 

 of form, the X ray being sometimes replaced by a cathode 

 ray, the 7 by a /3 ray. 



The argument that the X or 7 entity spends no energy 

 along its track arises simply from the fact that it produces a 

 cathode or a j3 ray of the same speed, no matter how much 

 material it has already traversed. It cannot keep its energy 

 intact while traversing matter and at the same time cause 

 ionization which involves the expenditure of energy. Gases 

 which are crossed by X and 7 rays are ionized, but that is 

 because they produce cathode and /3 rays respectively: and 

 these latter do the work. Of course it may be said that the 

 conversion of one X ray into one cathode ray is ionization: 

 and so it is ; but it is natural to keep this solitary and peculiar 

 event distinct from the general ionization of the gas along 

 the track of an entity. 



This deduction seems to afford an opportunity for putting 

 our hypothesis to the proof. What experiments have been 

 made from which we may determine whether or no X and 

 y rays ionize gases directly ? 



McLennan describes an experiment (Phil. Mag. Dec. 1907) 

 in which he shot 7 rays through two ionization-chambers, 

 one made of lead, the other of lead lined with aluminium, 



