September 13, 1912] 



SCIENCE 



329 



quantity generated is such that its energy 

 is the precise equivalent of the motive 

 power which might have been obtained if 

 the transfer of heat had been effected by- 

 means of a perfect engine working withovit 

 generation of caloric. The caloric gene- 

 rated in wasting a difference of tempera- 

 ture is the necessary and appropriate 

 measure of the quantity of heat obtained 

 by the degradation of available motive 

 power into the less available or transform- 

 able variety of heat energy. 



The processes by which caloric is gen- 

 erated in mixing substances at different 

 temperatures, or in other cases where avail- 

 able motive power is allowed to run to 

 waste, are generally of so turbulent a char- 

 acter that the steps of the process can not 

 be followed, although the final result can 

 be predicted under given conditions from 

 the energy principle. Such processes could 

 not be expected a priori to throw much 

 light on the nature of caloric. The fa- 

 miliar process of conduction of heat 

 through a body, the parts of which are at 

 different temperatures, while equally lead- 

 ing to the generation of a quantity of cal- 

 orie equivalent to the motive power wasted, 

 affords better promise of elucidating the 

 nature of caloric, owing to the comparative 

 simplicity and regularity of the phenom- 

 ena, which permit closer experimental 

 study. The earliest measurements of the 

 relative conducting powers of the metals 

 for heat and electricity showed that the 

 ratio of the thermal to the electric conduc- 

 tivity was nearly the same for all the pure 

 metals, and suggested that, in this case, the 

 carriers of heat and electricity were the 

 same. Later and more accurate experi- 

 ments showed that the ratio of the conduc- 

 tivities was not constant, but varied nearly 

 as the absolute temperature. At first sight 

 this might appear to suggest a radical dif- 

 ference between the two conductivities, but 



it results merely from the fact that heat is 

 measured as energy in the definition of 

 thermal conductivity, whereas electricity is 

 measured as a quantity of fluid. If ther- 

 mal conductivity were defined in terms of 

 caloric or thermal fluid, the ratio of the two 

 conductivities would be constant with re- 

 spect to temperature almost, if not quite, 

 within the limits of error of experiment. 

 On the hypothesis that the carriers are the 

 same for electricity and heat, and that the 

 kinetic energy of each carrier is the same 

 as that of a gas molecule at the same tem- 

 perature,' it becomes possible, on the anal- 

 ogy of the kinetic theory of gases, to cal- 

 culate the actual value of the ratio of the 

 conductivities. The value thus found 

 agrees closely in magnitude with that given 

 by experiment, and may be regarded as 

 conflrming the view that the carriers are 

 the same, although the hypotheses and 

 analogies invoked are somewhat specula- 

 tive. 



When the electrons or corpuscles of nega- 

 tive electricity were discovered it was a 

 natural step to identify them with the car- 

 riers of energy, and to imagine that a metal 

 contained a large number of such cor- 

 puscles, moving in all directions, and col- 

 liding with each other, and with the metal- 

 lic atoms, like the molecules of a gas on the 

 kinetic theory. If the mass of each carrier 

 were M.700 of that of an atom of hydrogen, 

 the velocity at 0° C. would be about sixty 

 miles a second, and would be of the right 

 order of magnitude to account for the ob- 

 served values of the conductivities of good 

 conductors, on the assumption that the 

 number of negative corpuscles was the 

 same as the number of positive metallic 

 atoms, and that the mean free path of each 

 corpuscle was of the same order as the dis- 

 tance between the atoms. The same hy- 

 pothesis served to give a qualitative ac- 

 count of thermo-electric phenomena, such 



