328 Prof. Brags: and Mr. Kleeman 



on 



The conclusion is that the a particle spends its energy at a 

 rate which is approximately inversely proportional to the 

 square root of its velocity. 



§iii. 



From the results described above as well as from those de- 

 scribed in the previous papers (Phil. Mag., Dec. 1904) it will be 

 clear that the law o£ the absorption of the a rays is consider- 

 ably different from that which has been generally assumed 

 hitherto. It has been supposed that a stream of radiation 

 gradually diminishes in power as it penetrates any material 

 substance, the rate o£ loss being approximately expressible 

 by an " absorption coefficient," and the law being therefore 

 nearly exponential. It is true that it has been pointed out 

 that this coefficient is not constant for any one stream in any 

 one gas, but varies with the distance of the stream from the 

 source. If this variation were small, there might be some 

 ground for considering that the law was fundamentally 

 dependent on an exponential form, but varied from it on 

 account of some secondary cause. Asa matter of fact, how- 

 ever, the variations are far too great to permit of such a 

 supposition. 



The true state of affairs is very different. There is no such 

 absorption coefficient, nor any approach to an exponential 

 law. 



Each ol particle pursues a rectilinear course, no matter what 

 it encounters : it passes through all the atoms it meets, 

 whether they form part of a solid or a gas (or in all proba- 

 bility of a liquid), suffering no deflexion on account of any 

 encounter until, at any rate, it is very near the end of its 

 course. It loses energy as it goes, a portion of this at least 

 being spent in producing ions. This drain on its energy, at 

 least, robs it of its extraordinary powers of penetration and 

 ionization. 



Prom this it will be clear that there are two quite 

 independent characteristics of a stream of a radiation : one, 

 the number of a particles in it_, the other, the energy of their 

 motion. The latter may more or less be evenly distributed 

 amongst the various particles. It is possible for the latter of 

 these characteristics to vary considerably whilst the first does 

 not change at all. For example, in the curves described 

 above, the a particles from Ra C remain unchanged in number 

 up to nearly 6*5 cms. from the commencement of their 

 motion ; but their energy has almost vanished at the longer 

 distance. Again, as will be seen later, a thin metal plate 



