1911] on a Fourth State of Matter 5 



slows down and spends more time in each atom, and considering^ the 

 more broken nature of the path near its end, the reason of these 

 peculiarities is clear enough. Apart from its comparative simplicity, 

 there are some other very interesting features of the particle's 

 motion. It is found, for example, that the loss of energy which the 

 particle incurs in crossing an atom is proportional to the square root 

 of the atomic weight very nearly, and there is no certain explanation 

 as yet of this curious law. And again, Gleiger has examined the 

 small scattering that does occur, and found that a particles when 

 moving cjuickly may be swung round completely even by the 

 thinnest films of gold leaf, though the number is so small that the 

 efiFect would have remained undetected had it not been for the 

 scintillation method which he and Rutherford have perfected. He 

 has found that about one particle in 8000 is returned in this way 

 from a gold plate, which need consist only of a few thicknesses of 

 gold leaf in order to give the maximum effect. 



Now let us take an example from the behaviour of the ^ rays. 

 The y8 particle is so light that it is easily deflected, even though it 

 moves several times as fast as the heavier a particle. Because it there- 

 fore possesses little energy its effects are much smaller, and no one has 

 yet succeeded in handling a single yS particle in the same way as 

 Rutherford and Geiger have handled the other. We are obliged to 

 content ourselves with observations of the effects of a crowd of /? 

 particles, since the combined action of many is necessary to give us 

 an observable result. And at the same time that the ^ particle gives 

 much less effect than the a, it has a much more irregular course, so 

 that the problem is doubly difficult. We are, in fact, only just 

 beginning to understand it. There is a compensation in the fact that 

 its very liabihty to deflection makes it all the more interesting an 

 object. It is possible — and this is the particular /3-ray problem 1 

 wish to consider now — to examine the deflection of a single y8 particle 

 by a single atom : the parallel result in the kinetic theory of gases 

 has never, of course, been achieved. 



Suppose that we project a stream of ^ rays against a thin plate 

 and measure the relative number sent back, which we do by measur- 

 ing the ionisations caused by the incident and returned rays respec- 

 tively. We do this for varying thicknesses of the plate, and plot the 

 results, as, for example, Madsen has done. His plate was made of 

 gold leaves, which could be had of extreme fineness. From the rela- 

 tion thus obtained, it is possible to obtain with confidence the amount 

 of /? radiation that would be returned by the thinnest plate that could 

 be imagined, only one molecule thick. In such case the particles 

 turned back could have had but one collision, and we have achieved 

 our purpose. Madsen's figures show that a plate weighing 4 mmg. 

 to the square centimetre turned back a tenth of the molecules that fell 

 upon it, and as far as can be judged the ratio of the proportion turned 

 back to the weight of the plate would be almost doubled for very thin 



