September 5, 



1912] 



NATURE 



25 



rent of heat in a metal are very closely related to the 

 corpuscles of electricity, and have an equal right to 

 be rvifarded as constituting a material fluid possessing 

 an objective physical existence. 



If I may be allowed to speculate a little on my own 

 account (as we are all here together in holiday mood, 

 and you will not take anything I may say too 

 seriously), I should prefer to regard the molecules of 

 caloric, not as being identical with the corpuscles of 

 negative electricity, but as being neutral doublets 

 formed by the union of a positive and negative cor- 

 puscle, in much the same way as a molecule of 

 hydrogen is formed by the union of two atoms. 

 Nothing smaller than a hydrogen atom has yet, so 

 far as I know, been discovered with a positive charge. 

 This may be merely a consequence of the limitations 

 of our experimental methods, which compel us to 

 employ metals to so large an extent as electrodes. 

 In the symmetry of nature it is almost inconceivable 

 that the positive corpuscle should not exist, if only as 

 the other end of the Faraday-tube or vorte.x-filament 

 representing a chemical bond. Prof. Bragg has 

 identified the X or 7 rays with neutral corpuscles 

 travelling at a high velocity, and has maintained this 

 hypothesis with brilliant success against the older 

 view that these rays are not separate entities, but 

 merely thin, spreading pulses in the sether produced 

 by the collisions of corpuscles with matter. I must 

 leave him to summarise the evidence, but if neutral 

 corpuscles exist, or can be generated in any way, it 

 should certainly be much easier to detach a neutral 

 corpuscle from a material atom or molecule than to 

 detach a corpuscle with a negative charge from the 

 positive atom with which it is associated. We should 

 therefore expect neutral corpuscles to be of such ex- 

 ceedingly common and universal occurrence that their 

 very existence might be overlooked, unless they hap- 

 pened to be travelling at such exceptionally high 

 velocities as are associated with the -y rays. Accord- 

 ing to the pulse theory, it is assumed that all 7 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 /3 rays, is a question of size, velocity, 

 and electric charge. Particles carrying electric 

 charges, like the « and |8 rays, lose energy in pro- 

 ducing ions by their electric field, perhaps without 

 actual collision. Neutral or 7 rays do not produce 

 ions directly, but dislodge either 7 rays or /3 rays from 

 atoms by direct collisions, which are comparatively 

 rare. The 3 rays alone, as C. T. R. Wilson's photo- 

 graphs show, are responsible for the ionisation. Per- 

 sonally, I have long been a convert to Prof. 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, permissible to assume their 

 existence for purposes of argument, in order to see 

 whether the conception may not be useful in the inter- 

 pretation of physical phenomena. 



If, for instance, we assume that the neutral cor- 

 puscles or molecules of caloric exist in conductors and 

 metallic bodies in a comparatively free state of solu- 

 tion, 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 directly on the analogy of conduc- 

 tion in electrolytic solutions. But, whereas in elec- 

 trolytes the ions are material atoms moving through a 

 viscous medium with comparatively 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 assump- 

 NO. 2236, VOL. 90] 



tions 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 electro- 

 lytic analogy, that the dissociation is practically com- 

 plete, 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 electricity. 

 But in bad conductors the dissociation will be far 

 from complete, and it is possible to see why, for 

 instance, the electric resistance of cast-iron should be 

 nearly ten times that of pure iron, although there is 

 comparatively little difference in their thermal con- 

 ductivities. The numerical magnitude of the thermo- 

 electric effect, which is commonly quoted in explana- 

 tion of the deviation of alloys from the electronic 

 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 conductivities. 



One of the oldest difficulties of the material theory 

 of heat is to explain the process of the production of 

 heat by friction. The application of the general prin- 

 ciple of the conservation of energy leads to the 

 undoubted conclusion that the thermal energy 

 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 

 regard to the actual nature of the energy produced in 

 the form of heat. It follows from the energy principle 

 that the quantity of caloric generated in the process 

 is such that its total energy at the final temperature 

 is equal to the work spent. If a quantity of caloric 

 represents so many neutral molecules of electricity, one 

 cannot help asking where they came from, and how 

 they were produced. It is certain that in most cases 

 of friction, wherever slip occurs, some molecules are 

 torn apart, and the work spent is represented in the 

 first instance by the separation of electric ions. Some 

 of these ions are permanently separated as frictional 

 electricity, and can be made to perform useful work ; 

 but the majority recombine before they can be effec- 

 tively separated, leaving only their equivalent in 

 thermal energy. The recombination of two ions is 

 generally regarded simply as reconstituting the 

 original molecule at a high temperature, but in the 

 light of recent discoveries we may perhaps go a step 

 further. It is generally admitted that X or 7 rays are 

 produced by thesudden stoppage of a charged corpuscle, 

 and Lorentz, in his electron theory of radiation, has 

 assumed that such is the case however low the velocity 

 of the electron. .\ similar effect must occur in the 

 sudden stoppage of a pair of ions rushing together 

 under the influence of their mutual attraction. Rays 

 produced in this way would be of an exceedingly soft 

 or absorbable character, but they would not differ in 

 kind from those produced by electrons except that 

 their energy, not exceeding that of a pair of ions, 

 would be too small to produce ionisation, so that they 

 could not be detected in the usual way. If the X rays 

 are corpuscular in their nature, we cannot logically 

 deny the corpuscular character even to the slowest 

 moving rays. We know that X rays continually pro- 

 duce other X rays of lower velocity. The final stage 

 is probably reached when the average energy of an 

 X corpuscle or molecule of caloric is the same as that 

 of a gas molecule at the same temperature, and the 

 number of molecules of caloric generated is such that 

 their total energy is equal to the work originally spent 

 in friction. 



In this connection it is interesting to note that Sir 

 J. J. Thomson, in a recent paper on ionisation by 

 m.oving particles, has arrived, on other grounds, at 



