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PRESIDENTIAL ADDRESS. 397 
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 Moving Particles,’ has arrived, on other grounds, at 
the conclusion that the character of the radiation emitted during the recombina- 
tion of the ions will be a series of pulses, each pulse containing the same amount 
of energy and being of the same type as very soft X rays. If the X rays are 
really corpuscular, these definite units or quanta of energy generated by the 
recombination of the ions bear a close resemblance to the hypothetical molecules 
of caloric. 
It may be objected that in many cases of friction, such as internal or viscous 
friction in a fluid, no electrification or ionisation is observable, and that the 
generation of caloric cannot in this case be attributed to the recombination of 
ions. It must, however, be remarked that the generation of a molecule of 
caloric requires less energy than the separation of two ions; that, just as the 
separation of two ions corresponds with the breaking of a chemical bond, so 
the generation of one or more molecules of caloric may correspond with the 
rupture of a physical bond, such as the separation of a molecule of vapour 
from a liquid or solid. The assumption of a molecular constitution for caloric 
follows almost of necessity from the molecular theories of matter and electricity, 
and is not inconsistent with any well-established experimental facts. On the 
contrary, the many relations which are known to exist between the specific 
heats of similar substances, and also between the latent heats, would appear to 
lead naturally to a molecular theory of caloric. For instance, it has often been 
noticed that the molecular latent heats of vaporisation of similar compounds 
at their boiling-points are proportional to the absolute temperature. It follows 
that the molecular latent caloric of vaporisation is the same for all such com- 
pounds, or that they require the same number of molecules of caloric to effect the 
same change of state, irrespective of the absolute temperatures of their boiling- 
points. From this point of view one may naturally regard the liquid and gaseous 
states as conjugate solutions of caloric in matter and matter in caloric respec- 
tively. The proportion of caloric to matter varies regularly with pressure and 
temperature, and there is a definite saturation limit of solubility at each tem- 
perature. 
One of the most difficult cases of the generation of caloric to follow in detail 
is that which occurs whenever there is exchange of heat by radiation between 
bodies at different temperatures. If radiation is an electro-magnetic wave- 
motion, we must suppose that there is some kind of electric oscillator or 
resonator in the constitution of a material molecule which is capable of respond- 
ing to the electric oscillations. If the natural periods of the resonators 
correspond sufficiently closely with those of the incident radiation the ampli- 
tude of the vibration excited may be sufficient to cause the ejection of a 
corpuscle of caloric. It is generally admitted that the ejection of an electron 
may be brought about in this manner, but it would evidently require far less 
energy to produce the emission of a neutral corpuscle, which ought therefore 
to be a much more common effect. On this view, the conversion of energy of 
radiation into energy of caloric is a discontinuous process taking place by definite 
molecular increments, but the absorption or emission of radiation itself is a con- 
tinuous process. Professor Planck, by a most ingenious argument based on 
the probability of the distribution of energy among a large number of similar 
electric oscillators (in which the entropy is taken as the logarithm of the 
probability, and the temperature as the rate of increase of energy per unit of 
entropy), has succeeded in deducing his well-known formula for the distribu- 
tion of energy in full radiation at any temperature; and has recently, by a 
further extension of the same line of argument, arrived at the remarkable con- 
clusion that, while the absorption of radiation is continuous, the emission of 
radiation is discontinuous, occurring in discrete elements or quanta. Where 
an argument depends on so many intricate hypotheses and analogies the possible 
interpretations of the mathematical formule are to some extent uncertain; 
but it would appear that Professor Planck’s equations are not necessarily 
inconsistent with the view above expressed, that both emission and absorption of 
