September 13, 1912] 



SCIENCE 



331 



that their very existence might be over- 

 looked, unless they happened to be travel- 

 ing at such exceptionally high velocities as 

 are associated with the y rays. According 

 to the pulse theory, it is assumed that all y 

 rays travel with the velocity of light, and 

 that the enormous variations observed in 

 their penetrative power depend simply on 

 the thickness of the pulse transmitted. On 

 the corpuscular theory, the penetrative 

 power, like that of the a and p rays, is a 

 question of size, velocity, and electric 

 charge. Particles carrying electric charges, 

 like the a and /? rays, lose energy in pro- 

 ducing ions by their electric field, perhaps 

 without actual collision. Neutral or y rays 

 do not produce ions directly, but dislodge 

 either y rays or ^ rays from atoms by di- 

 rect collisions, which are comparatively 

 rare. The /8 rays alone, as C. T. R. "Wilson's 

 photographs show, are responsible for the 

 ionization. Personally, I have long been a 

 convert to Professor Bragg 's views on the 

 nature of X rays, but even if we regard 

 the existence of neutral corpuscles as not 

 yet definitely proved, it is, I think, per- 

 missible to assume their existence for pur- 

 poses of argTiment, in order to see whether 

 the conception may not be useful in the 

 interpretation of physical phenomena. 



If, for instance, we assume that these 

 neutral corpuscles or molecules of caloric 

 exist in conductors and metallic bodies in a 

 comparatively free state of solution, and 

 are readily dissociated into positive and 

 negative electrons owing to the high specific 

 inductive capacity of the medium, the whole 

 theory of metallic conduction follows di- 

 rectly on the analogy of conduction in 

 electrolytic solutions. But, whereas in elec- 

 trolytes the ions are material atoms moving 

 through a viscous medium with compara- 

 tively low velocities, the ions in metallic 

 conductors are electric corpuscles moving 



with high velocities more after the manner 

 postulated in the kinetic theory of gases. 

 It is easy to see that this theory will give 

 similar numerical results to the electronic 

 theory when similar assumptions are made 

 in the course of the work. But it has the 

 advantage of greater latitude in explaining 

 the vagaries of sign of the Hall effect, and 

 many other peculiarities in the variation of 

 resistance and thermo-electric power with 

 temperature. For good conductors, like 

 the pure metals, we may suppose, on the 

 electrolytic analogy, that the dissociation is 

 practically complete, so that the ratio of 

 the conductivities will approach the value 

 calculated on the assumption that all the 

 carriers of heat are also carriers of elec- 

 tricity. But in bad conductors the disso- 

 ciation wiU be far from complete, and it is 

 possible to see why, for instance, the elec- 

 tric resistance of cast iron should be nearly 

 ten times that of pure iron, although there 

 is comparatively little difference in their 

 thermal conductivities. The numerical 

 magnitude of the thermo-electric effect, 

 which is commonly quoted in explanation 

 of the deviation of alloys from the elec- 

 tronic theory, is far too small to produce 

 the required result ; and there is little or no 

 correspondence between the thermo-electric 

 properties of the constituents of alloys and 

 the variations of their electric conductivi- 

 ties. 



One of the oldest difficulties of the ma- 

 terial theory of heat is to explain the proc- 

 ess of the production of heat by friction. 

 The application of the general principle of 

 the conservation of energy leads to the 

 undoubted conclusion that the thermal en- 

 ergy generated is the equivalent of the 

 mechanical work spent in friction, but 

 throws little or no light on the steps of the 

 process, and gives no information with re- 

 gard to the actual nature of the energy 



