814 TRANSACTIONS OF SECTION G. 



is possible, because the same quantity of heat is supposed to exist at the low as at 

 the high temperature. On this theory nothing in the idea of temperature suggests 

 a possible physical limit. On the material theory, the notion of temperature is one 

 to which it is exceedingly difficult to attach a precise meaning. 



Thomson's promises of further investigation were fulfilled in 1850, in which 

 year he definitely accepted the dynamical theory of heat and finally abandoned the 

 material. His conclusions are given in a memoir of the first importance which 

 was read before the Royal Society of Edinburgh in 1851. It was entitled ' On 

 the Dynamical Theory of Heat.' Before dealing with it, however, it is desirable 

 to consider the work of Joule and others on another side of thermodynamics. 



Long before 1850 the equivalence of mechanical work and heat quantity had 

 been accepted by many scientific men, and Eumford had, indeed, made measure- 

 ments of a rough kind. It remained, however, for Joule experimentally to 

 determine the mechanical equivalent in the most accurate manner and place what 

 is now known as the first law of thermodynamics upon the sure basis of absolute 

 experimental determination. His first paper was read before the Cork Meeting 

 of the British Association in 1843, and at the Oxford Meeting in 1847 he read 

 another — ' On the Mechanical Equivalent of Heat ' — describing the results of 

 experiments with paddles rotating in liquids driven by falling weights. By 

 these years of work he had absolutely demonstrated the equivalence of heat 

 quantity and mechanical work, so that no loophole of escape seemed possible ; it 

 appeared as if the material theory was rendered intellectually impossible to the 

 trained intellect. This was not the fact, however, as is evident from both Joule's 

 and Thomson's accounts of that British Association Meeting. 



.Joule's earlier paper had been coolly received. Indeed, it is evident that the 

 idea of a mechanical equiv^alent of heat was still distasteful to the physicists of 

 the day, and its discussion was looked upon with dislike. Joule, at the 1847 

 Meeting, addressed a small audience, and the account of his experiments was 

 received without enthusiasm. This adverse atmosphere, so discouraging to the 

 investigator, was quickly removed, however, when a young man rose to make his 

 remarks, and, by his enthusiastic comment and clear reasoning, at once succeeded 

 in attracting the interest of those present. This young man was William 

 Thomson, Professor of Natural Pliilosophy in the University of Cilasgow. 

 Speaking of this, his first meeting with Joule, at Manchester forty-six years later. 

 Lord Kelvin said: ' I can never forget the British Association at Oxford in the 

 year 1847, when in one of the Sections I heard a paper read by a very unassuming 

 young man, who betrayed no consciousness in his manner that he had a great idea 

 to unfold. I was tremendously struck with the paper. I had first thought it 

 could not be true because it was different from Carnot's theory, and immediately 

 after the reading of the paper I had a few words of conversation with the author, 

 James Joule, which was the beginning of our forty years' acquaintance and 

 friendship. ... I gained ideas which had never entered my mind before, and 

 I thought I, too, suggested something worthy of .Joule's consideration when I 

 told him of Carnot's theory.' This meeting was indeed fateful for the future of 

 the science of thermodynamics, as it resulted in co-operation between two men 

 of giant intellect, who between them performed most of the experimental work 

 which was necessary to make thermodynamics an exact science. Their work 

 alone sufficed to place the first and second laws of thermodynamics on the firm 

 footing of accurate experiment and logical deduction. 



Although Thomson was much struck by Joule's experiments, he did not 

 accept the dynamical theory of heat at once. As he stated himself : ' I had first 

 thought that it could not be true because it was different from Carnot's theory.' 

 Joule's discoveries at this date may be thus expressed : — 



Heat and mechanical energy are mutually convertible, and heat requires for 

 its production, and produces by its disappearance, mechanical energy in the pro- 

 portion of 1,390 foot-pounds for each centigrade heat unit, a heat unit being the 

 amount of heat necessary to heat one pound of water through 1° C. 



Knowing, as Thomson did, that mechanical energy could be produced by the 



