CONTEMPORARY ADVANCES IN PHYSICS 299 



When the screens were of metal, nothing sensational happened; 

 but if they were of paraffin, water or cellophane — materials containing 

 hydrogen — the ionization- cur rent went up instead of down. This was 

 not the first time that a screen had been observed to enhance the 

 effect of what supposedly were gamma-rays, but in the previous cases 

 it was permissible to infer that the rays were expelling electrons from 

 the substance of the screen. Here the substances were distinguished 

 not by abundance of electrons, but by abundance of hydrogen atoms in 

 their structure; and Curie and Joliot conceived the idea that the 

 primary rays were ejecting protons from the screen, which entered 

 the chamber and in it ionized abundantly. This theory they fortified 

 at once by applying magnetic fields, and finding that the ionization 

 persisted (electrons issuing from the paraffin would have been twisted 

 back, unless extremely fast); by interposing 0.2 mm. of aluminium, 

 and finding that the extra ionization ceased (electrons, if extremely 

 fast, might have got through) ; and by taking cloud-chamber photo- 

 graphs, and observing tracks of the aspect of proton-tracks springing 

 out of the paraffin and traversing the ionization-chamber partly or 

 altogether. 



At once it was guessed by Curie and Joliot that these protons were 

 recoiling from elastic impacts of the high-energy photons which the 

 primary rays were still supposed to be — that they had suffered, in 

 fact, the very same sort of blow as electrons suffer in the well-known 

 "Compton effect." So great, however, was the energy of the protons 

 (as evinced by their range) that photons of energy almost incredibly 

 great had to be postulated ; such would probably have an even greater 

 penetrating power than that of the primary rays, and there were 

 other objections more or less solidly founded on theory, which now it 

 would be scarcely worth while to discuss. The French physicists were 

 aware of these difficulties, and published them; but it was reserved for 

 one of the Cavendish group to reject the idea altogether, and supplant 

 it with the one which at present is accepted. Chadwick seized upon 

 the revelations from the Institut du Radium with such alacrity that 

 within six weeks he was reporting data obtained by counters and by 

 cloud-chambers — data which confirmed that the rays emitted from 

 beryllium when bombarded by alpha-particles are able to confer great 

 speeds not only upon protons, but on nuclei of other elements of low 

 mass (a later list comprises Li, He, Be, B, C, N, O, A; and Kirsch has 

 very recently detected emission of neutrons from many more). Out of 

 these data emerges the fact which speaks most clearly for his theory 

 that the corpuscles which impel the protons and other nuclei are 

 material particles of nearly the mass of a proton, instead of being 

 corpuscles of light. 



