CONTEMPORARY ADVANCES IN PHYSICS 695 



and at the high density, few had been deflected enough to be caught by 

 the walls. 22 



G. P. Thomson developed another way of studying what happens 

 to a stream of protons shooting through a gas. He set a photographic 

 plate athwart the path of the narrow beam, and observed the imprint, 

 which grew broader when a gas was introduced into the path. By 

 measuring the darkening of the plate from point to point across the 

 imprint, he was able to deduce the law of scattering-in-angle of the 

 protons. The most interesting single feature of his data was a likeness 

 to Ramsauer's famous observation with electrons: using either argon 

 or helium, Thomson found that as the speed of the protons was de- 

 creased, the broadening of the beam rose to a maximum and then 

 declined. This implies that for either gas the cross-section for inter- 

 ception of protons has a maximum, like that for interception of elec- 

 trons. Moreover for either gas, the two cross-sections have their 

 maxima at about the same speed, though not the same kinetic energy — 

 owing to their difference in mass, a proton has some 1840 times as 

 much vis viva as an electron keeping pace with it. Those with which 

 Thomson was working had energy measured in thousands of volts. 



Much of the work of Kallmann and Rosen is directed to testing the 

 theorem stated above about the cross-section for electron-transfer. 

 This they succeed in doing on a number of cases, with favorable results. 

 Thus in nitrogen gas, the value of u is much greater for N2"'" ions than 

 for N+ ions; but in oxygen it is the N+ ion for which the value of a 

 is greater, for the ionizing potential of the N atom lies closer to that of 

 the O2 molecule than does that of the N2 molecule. Likewise Holzer 

 has found that in hydrogen, the cross-section for interception is greater 

 for H2+ than for either H+ or H3+. It is not certain in any of these 

 cases that the a measured refers entirely to interception by electron- 

 transfer, but the evidence nevertheless is strong. 



The scattering of exceedingly fast helium nuclei — alpha-particles — 

 is a subdivision of this field, — the best known, probably, of all, since 

 out of it the central feature of our contemporary theory of the atom 

 is derived. The value of cr for this scattering is vanishingly small, by 

 comparison even with the gas-kinetic cross-section. But since the 

 speed of the particles is so great, this result is for once exactly what 

 we should expect. 



2^ Dempster phrases the results in terms of the "mean free path," the reciprocal of 

 N(T {N standing for the number of atoms of gas per unit volume). The "high den- 

 sity" mentioned in this sentence was such, that the semi-circular path of the protons 

 between the two slits was 108 times as long as the quantity (iVcro)"' previously defined, 

 the "gas-kinetic mean free path." Dempster also observed the extinction of 

 a beam of H2'*" ions, accompanied by the advent of a beam of slow-moving H+ ions 

 probably due to the dissociation of the former. 



