PRESIDENTIAL ADDRESS. 399 
associated with a quantity of caloric must be so small that we could not hope to 
learn much about it by the direct use of the balance. 
The fundamental property of caloric, that its total quantity cannot be 
diminished by any known process, and that it is not energy, but merely the 
vehicle or carrier of energy, is most simply represented in thought by imagining 
it to consist of some indestructible form of matter. The further property, that 
it is always generated in any turbulent or irreversible process, appears at first 
sight to conflict with this idea, because it is difficult to see how anything inde- 
structible can be so easily generated. When, however, we speak of caloric as 
being generated, what we really mean is that it becomes associated with a material 
body in such a way that we can observe and measure its quantity by the change 
of state produced. The caloric may have existed previously in a form in which 
its presence could not be detected. In the light of recent discoveries we might 
suppose the caloric generated to arise from the disintegration of the atoms of 
matter. No doubt some caloric is produced in this way, but those corpuscles 
that are so strongly held as to be incapable of detection by ordinary physical 
methods require intense shocks to dislodge them. A more probable source of 
caloric is the zther, which, so iar as we know, may consist entirely of neutral 
corpuscles of caloric. The hypothesis of a continuous xther has led to great 
difficulties in the electro-magnetic theory of light and in the kinetic theory of 
gases. A molecular, or cellular-vortex, structure appears to be required. Accord- 
ing to the researches of Kelvin, Fitzgerald, and Hicks, such an ether can be 
devised to satisfy the requirements of the electro-magnetic theory without requir- 
ing it to possess a density many times greater than that of platinum. So far asthe 
properties of caloric are concerned, a neutral pair of electrons would appear to 
constitute the simplest type of molecule, though without more exact knowledge 
of the ultimate nature of an electric charge it would be impossible to predict 
all its properties. Whether an xther composed of such molecules would be com- 
petent to discharge satisfactorily all the onerous functions expected from it may 
be difficult to decide, but the inquiry, in its turn, would probably throw light 
on the ultimate structure of the molecule. 
Without venturing too far into the regions of metaphysical speculation, or 
reasoning in vicious circles about the nature of an electric charge, we may at 
least assert with some dezree of plausibility that material bodies under ordinary 
conditions probably contain a number of discrete physical entities, similar in kind 
to X rays or neutral corpuscles, which are capable of acting as carriers of energy, 
and of preserving the statistical equilibrium between matter and radiation at any 
temperature in virtue of their interchanges with electrons. If we go a step 
further and identify these corpuscles with the molecules of caloric, we shall cer- 
tainly come in conflict with some of the fundamental dogmas of the kinetic 
theory, which tries to express everything in terms of energy, but the change 
involved is mainly one of standpoint or expression. The experimental facts 
remain the same, but we describe them differently. Caloric has a physical 
existence, instead of being merely the logarithm of the probability of a com- 
plexion, In common with many experimentalists, I cannot help feeling that we 
have everything to gain by attaching a material conception to a quantity of caloric 
as the natural measure of a quantity of heat as opposed to a quantity of heat 
energy. In the time at my disposal I could not pretend to offer you more than 
a suggestion of a sketch, ‘an apology for the possibility of an explanation, but 
I hope I may have succeeded in conveying the impression that a caloric theory of 
heat is not so entirely unreasonable in the light of recent experiment as we are 
sometimes led to imagine. 
The following Papers were then read :— 
1. The Heating Effect of Radium Emanation and its Products. 
By Professor E. Ruruerrorp, F.R.S., and H. Rostnson. 
