398 TRANSACTIONS OF SECTION A. 
radiation are continuous, and that his elementa quanta, the energy of which 
varies with their frequency, should rather be identified with the molecules of 
caloric, representing the conversion of the electro-magnetic energy of radiation 
into the form of heat, and possessing energy in proportion to their temperature. 
Among the difficulties felt rather than explicitly stated, in regarding entropy 
or caloric as the measure of heat quantity, is its awkward habit of becoming 
infinite, according to the usual approximate formule, at extremes of pressure 
or temperature. If caloric is to be regarded as the measure of heat quantity, 
the quantity existing in a finite body must be finite, and must vanish at the 
absolute zero of temperature. In reality there is no experimental foundation 
for any other conclusion. According to the usual gas formule, it would be 
possible to extract an infinite quantity of caloric from a finite quantity of gas 
by compressing it at constant temperature. It is true that (even if we assumed 
the law of gases to hold up to infinite pressures, which is far from being the 
case) the quantity of caloric extracted would be of an infinitely low order of 
infinity as compared with the pressure required. But, as a matter of fact, 
experiment indicates that the quantity obtainable would be finite, although its 
exact value cannot be calculated owing to our ignorance of the properties of 
gases at infinite pressures. In a similar way, if we assume that the specific 
heat as ordinarily measured remains constant, or approaches a finite limit at 
the absolute zero of temperature, we should arrive at the conclusion that an 
infinite quantity of caloric would be required to raise the temperature of a finite 
body from 0° to 1° absolute. The tendency of recent experimental work on 
specific heats at low temperatures, by Tilden, Nernst, Lindemann, and others, is 
to show, on the contrary, that the specific heats of all substances tend to vanish 
as the absolute zero is approached and that it is the specific capacity for caloric 
which approaches a finite limit. The theory of the variation of the specific 
heats of solids at low temperatures is one of the most vital problems in the 
theory of héat at the present time, and is engaging the attention of many active 
workers. Professor Lindemann, one of the leading exponents of this work, has 
kindly consented to open a discussion on the subject in our Section. We are 
very fortunate to have succeeded in securing so able an exponent, and shall 
await his exposition with the greatest interest. For the present I need only add 
that the obvious conclusion of the caloric theory bids fair to be completely 
justified. 
A most interesting question, which early presented itself to Rumford and 
other inguirers into the caloric theory of heat, was whether caloric possessed 
weight. While a positive answer to this question would be greatly in favour of a 
material theory, a négative answer, such as that found by Rumford, or quite 
recently by Professor Poynting and Phillips, and by Mr. L. Southerns working 
independently, would not be conclusively against it. The latter observers found 
that the change in weight, if any, certainly did not exceed 1 in 10° per 1° C. Tf 
the mass of a molecule of caloric were the same as that generally attributed to an 
electron, the change of weight, in the cases tested, should have been of the order 
of 1 in 10’ per 1° C., and should not have escaped detection. It is generally 
agreed, however, that the mass of the electron is entirely electro-magnetic. Any 
such statement virtually assumes a particular distribution of the electricity in a 
spherical electron of given size. But if electricity itself really consists of electrons, 
an argument’ of this type would appear to be so perfectly circular that it is ques- 
tionable how much weight should be attached to it. If the equivalent mass of 
an electron in motion arises solely from the electro-magnetic field produced by its 
motion, a neutral corpuscle of caloric should not possess mass or energy of trans- 
lation as a whole, though it might still possess energy of vibration or rotation 
of its separate charges. For the purpose of mental imagery we might picture the 
electron as the free or broken end of a vortex filament, and the neutral corpuscle 
as a vortex ring produced when the positive and negative ends are united; but a 
mental picture of this kind does not carry us any further than the sphere coated 
with electricity, except in so far as either image may suggest points for experi- 
mental investigation. In our ignorance of the exact mechanism of gravity it is 
even conceivable that a particle of caloric might possess mass without possessing 
weight, though, with the possible exception of the electron, nothing of the kind 
has yet been demonstrated. In any case it would appear that the mass, if any, 
