rUESlDENTIAL ADDRESS. 815 



agency of lieat, but that its amount varied with the temperature and temperature 

 fall, Joule's discoveries seemed antagonistic to Carnot's demonstration ; and, con- 

 vinced as he was that Carnot's law was true, he naturally felt at first that there 

 must be some other way of looking at Joule's results than that adopted by Joule 

 himself. 



Joule naturally believed in his own manner of looking at his results, and he 

 apparently agreed with Thomson as to the antagonism between what may be here 

 called the Carnot and Joule laws. 



The material theory of heat might have been true ; in which case there was 

 no more need for any direct quantitative connection between heat quantity and 

 mechanical energy than between the mass of a body and its mechanical energy. 

 Any unit of mass may acquire any conceivable amount of mechanical energy if 

 its velocity be great enough, and so any unit of heat on the caloric theory may 

 produce any conceivable amount of mechanical energy if the temperature fall be 

 great enough. Joule considered the Carnot law to be so inconsistent with his 

 law that in one of his papers he proposes its abandonment as inconsistent with 

 discovered facts. At this point the two ideas seem to be in opposition. The 

 germ of reconciliation, however, is found in observations by Thomson in both the 

 1848 and 1849 papers. In paragraph 8, quoted here from the latter papei-, it is 

 stated : — 



' In the present state of science, however, no operation is known by which 

 heat can be absorbed into a body without either elevating its temperature or 

 becoming latent and producing some alteration in its physical condition.' 



This is equivalent to saying that no case has been observed where heat dis- 

 appears doing mechanical work. In a note occurring in the same paper he 

 alludes to the fact that engineers always assume that the amount of heat found 

 in the condenser of the steam engine was the same as that taken into the engine 

 by the steam, in the following terms : — 



'So generally is Carnot's principle tacitly admitted as an axiom that its 

 application in this case has never, so far as 1 am aware, been questioned by 

 practical engineers.' 



This was quite accurate. Hirn's demonstrations that heat disappears in a 

 steam engine when work is done was not made until 1857, eight years later. 



In the 1848 paper he states : — 



' The experiments of Mr. Joule of Manchester seem to indicate an actual con- 

 version of mechanical eifect into caloric. No experiment, however, is adduced in 

 which the converse operation is exhibited ; but it must be confessed that as yet 

 much is involved in mystery with reference to these fundamental questions of 

 natural philosophy.' 



Here we find Thomson's mind engaged — in 1848 and 1849 — with the very 

 matter requiring proof. Joule had proved the generation of heat by means of 

 mechanical work ; Thomson required the i^rouf of the converse case — the dis- 

 appearance of heat when mechanical work was done by the working fluid. 



This proof was forthcoming in the results of experiments on the compression 

 and expansion of air. Accordingly, we find the Carnot and Joule principles re- 

 conciled in Thomson's paper of 1861, and the important deduction made of an 

 absolute zero of temperature at —273° on the Centigrade scale. The introduction 

 of the idea of the mechanical equivalent of heat leads at once to an absolute 

 zero of temperature, and allows of the determination of this physical lower limit 

 by the use of the Carnot cj'cle for investigating the efficiency of a perfect engine 

 using any working fluid. Air was the working fluid actually investigated, and 

 the determination of its properties at ordinary temperatures was a vitally important 

 result of the co-operation of Thomson and Joule. Their experiments lasted for 

 many years, and their rigorous investigation disclosed the fact that internal work 

 was done in expanding a gas ; in fact, that in a gas expanding isothermally doing 

 work, part of the heat only disappeared in external work and part was absorbed 

 in separating the molecules, 



