September 24, 1908] 



NATURE 



521 



rough kind. It remained, however, for Joule experi- 

 mentally to determine the mechanical equivalent in the 

 most accurate manner and place what is now known as 

 the tirst law of thermodynamics upon the sure basis of 

 absolute experimental determination. His first Paper was 

 read before the Coric Meeting of the British Association 

 in 1S43, and at the Oxford Meeting in 1S47 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 worli 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 equivalent 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 Philosophy in the Uni- 

 versity of Glasgow. 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 some- 

 thing worthv of Joule's consideration when I told him 

 of Carnot's theory." This Meeting was indeed fateful for 

 the future of the science of tliermodynamics, as it resulted 

 in cooperation 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 e.xperiment and logical deduction. 



-Although Thomson was much struck by Joule's experi- 

 ments, 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 bv its 

 disappearance, mechanical energy in the proportion of 1300 

 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 agency of heat, but that its 

 amount varied with the temperature and temperature fall. 

 Joule's discoveries seemed antagonistic to Carnot's demon- 

 stration; and, convinced as he w^as 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 quantita- 

 tive 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 



NO. 2030, VOL. 78] 



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 incon- 

 sistent 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 

 1S48 and 1849 Papers. In paragraph 8, quoted here from 

 the latter Paper, 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 disappears 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 generallv is Carnot's principle tacitly admitted as 

 an axiom that its applic^ition in tltis case has never, so 

 far as I am aware, been questioned by practical engineers." 



This was quite accurate. Hirn's demonstration 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 conversion of mechanical effect 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 

 1840 — with the very matter requiring proof. Joule had 

 proved the generation of heat by means of mechanical 

 work ; Thomson required the proof of the converse case — 

 the disappearance 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. .Arcordingly, 

 we find the Carnot and Joule principles reconciled in 

 Thomson's Paper of 1S51, and the important deduction 

 made of an absolute zero of temperature at — 27.5° 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 cycle 

 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 

 ordinarv temperatures was a vitally important result of 

 the cooperation of Thomson and Joule. Their experiments 

 lasted for many years, and their rigorous investigation 

 disclosed the fact that internal work was done in expand- 

 ing 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. 



The Joule and Carnot laws are now known as the first 

 and second laws of thermodynamics. 



The second law, in modern form, may be thus stated : — 



.Although heat and work are mutually convertible and 

 in definite and invariable proportions, yet no conceivable 

 heat engine is able to convert all the heat given to it into 

 work. Apart altogether from practical limitations, a 

 certain portion of the heat must be passed from the hot 

 body to the cold body in order that the remainder may 

 assume the form of mechanical energy. 



The proportion of the total heat convertible into 

 mechanical energy depends on the absolute temperatures 

 of the hot and cold bodies; it is unity minus the lower 

 absolute temperature upon the upper absolute temperature. 



It appears that during Thomson's struggle to reconcile 

 the two apparently opposing laws, Clausius, who had 

 seen the same difficulty, arrived independentlv at its solu- 

 tion and published a Paper, " On the Motive Power of 

 Heat and the Laws of Heat which may be deduced there- 

 from," at the Berlin .Academy in February. 1850. In 

 this Paper, Clausius discusses Thomson's difiiculties, and 



