p PRESIDENTIAL ADDRESS. 389 
impression that there is something fundamentally wrong about the caloric theory, 
and that it is in the nature of things incapable of correctly representing the facts. 
I shall endeavour to show that this fictitious antagonism between the two theories 
is without real foundation. They should rather be regarded as different ways 
of describing the same phenomena. Neither is complete without the other. The 
kinetic theory is generally preferable for elementary exposition, and has come 
to be almost exclusively adopted for this purpose; but in many cases the caloric 
theory would have the advantage of emphasising at the outset the importance of 
fundamental facts which are too often obscured in the prevailing method of 
treatment. 
The explanation of the development of heat by friction was one of the 
earliest difficulties encountered by the caloric theory. One explanation, main- 
tained by Cavendish and others, was simply that caloric was generated de novo 
by friction in much the same way as electricity. Another explanation, more 
commonly adopted, was that the fragments of solid, abraded in such operations 
as boring cannon, had a smaller capacity for heat than the original material. 
Caloric already existing in the substance was regarded as being squeezed or 
ground out of it without any fresh caloric being actually generated. ‘lhe proba- 
bility of the second explanation was negatived by the celebrated experiments of 
Rumford and Davy, who concluded that friction did not diminish the capacities 
of bodies for heat, and that it could not be a material substance because the 
supply obtainable by friction appeared to be inexhaustible. Rumford also 
showed that no increase of weight in a body when heated could be detected by 
the most delicate apparatus available in his time. Caloric evidently did not 
possess to any marked extent the properties of an ordinary ponderable fluid; but, 
if it had any real existence and was not merely a convenient mathematical 
fiction, it must be something of the same nature as the electric fluids, which had 
already played so useful a part in the description of phenomena, although their 
actual existence as physical entities had not then been demonstrated. Heat, as 
Rumford and Davy maintained, might be merely a mode of motion or a 
vibration of the ultimate particles of matter, but the idea in this form was too 
vague to serve as a basis of measurement or calculation. The simple conception 
of caloric, as a measurable quantity of something, sufficed for many purposes, 
and led in the hands of Laplace and others to correct results for the ratio of the 
specific heats, the adiabatic equation of gases, and other fundamental points of 
theory, though many problems in the relations of heat and work remained 
obscure. 
The greatest contribution of the caloric theory to thermodynamics was the 
production of Carnot’s immortal ‘ Reflections on the Motive Power of Heat.’ It 
is one of the most remarkable illustrations of the undeserved discredit into which 
the caloric theory has fallen, that this work, the very foundation of modern 
thermodynamics, should still be misrepresented, and its logic assailed, on the 
ground that much of the reasoning is expressed in the language of the caloric 
theory. In justice to Carnot, even at the risk of wearying you with an oft-told 
tale, I cannot refrain from taking this opportunity of reviewing the essential 
points of his reasoning, because it affords incidentally the best introduction to the 
conception of caloric, and explains how a quantity of caloric is to be measured. 
At the time when Carnot wrote, the industrial importance of the steam-engine 
was already established, and the economy gained by expansive working was 
generally appreciated. The air-engine, and a primitive form of the internal-com- 
bustion engine, had recently been invented. On account of the high value of the 
latent heat of steam, it was confidently expected that more work might be 
obtained from a given quantity of heat or fuel by employing some other working 
substance, such as alcohol or ether, in place of steam. Carnot set himself to 
investigate the conditions under which motive-power was obtainable from heat, 
how the efficiency was limited, and whether other agents were preferable to 
steam. These were questions of immediate practical importance to the engineer, 
but the answer which Carnot found embraces the whole range of science in its 
ever-widening scope. ¥ } 
In discussing the production of work from heat it is necessary, as Carnot 
points out, to consider a complete series or cycle of operations in which the 
working substance, and all parts of the engine, are restored on completion of the 
