522 



NA TURE 



[September 24, 1908 



.also arrives at the conclusion that the Carnot cycle may 

 be reconciled to Joule's law by the omission of the sup- 

 position that during the third process the same amount 

 uf heat is discharged from the cool body as was taken in 

 from the hot one. He states : — 



" On a nearer view of the case we find that the new 

 theories were opposed not to the real fundamental prin- 

 ciple of Carnot, but to the addition that no heat is lost. 

 For it is quite possible that in the production of work 

 both may take place at the same time : a certain portion 

 of heat may be consumed and a further portion transmitted 

 from a warm body to a cold one ; and both portions may 

 stand in a certain definite relation to the quantity of 

 work produced. This will be made plainer as we proceed ; 

 and it will be moreover shown that the inference to be 

 ■drawn from both assumptions may not only exist together, 

 but that they may mutually support each other." 



In his 185 1 Paper, Thomson gives Clausius full credit 

 for solving the difiiculty between the Carnot and the Joule 

 principles. Thomson gives Clausius the full credit for 

 priority, but states that he was working on the same 

 problem and had arrived at the same solution in the year 

 1850, before he had seen Clausius' work. Clausius, how- 

 ever, assumed the theory of a permanent gas, which re- 

 quired the absence of internal work, but Thomson was 

 not prepared to assume this without experiment. This 

 determination rigidly to prove every necessary assumption, 

 and his clear conception of the points necessary for proof, 

 .led to the extensive series of researches undertaken by 

 Thomson and Joule with the object of determining how 

 much gas thermometers differ from an absolute scale as 

 determined by the combination of the Joule and Carnot 

 laws. 



Rankine, as early as 1849, arrived at the general equa- 

 tion of thermodynamics which expresses the relation 

 between heat and mechanical energy, and indicated the 

 result of his investigations to the Royal Society of Edin- 

 burgh in February, 1850. Rankine thus arrived independ- 

 ently at the same result as Clausius about the same time. 

 Both Rankine and Clausius, however, adopted certain 

 theories as to the molecular structures and motions of 

 :gases, and their demonstrations to some extent depended 

 upon their theories. To Thomson and Joule we are deeply 

 indebted for the rigid proof of the two laws and for the 

 rigid deduction of the modern scale of temperature and 

 the determination of absolute zero in its modern form. 

 Thomson now thus defines temperature : — 



" The temperatures of two bodies are proportional to 

 the quantities of heat respectively taken in and given out 

 in localities at one temperature and at the other respec- 

 tively, by a material system subjected to a complete cycle 

 •of perfectly reversible thermodynamic operations, and not 

 allowed to part with or take in heat at any other tempera- 

 ture ; or, the absolute values of two temperatures are to 

 ■one another in the proportion of the heat taken in to 

 the heat rejected in a perfect thermodynamic engine, 

 working with a source and refrigerator at the higher and 

 lower of the temperatures respectively." 



This definition leads to an absolute scale of temperature 

 which is independent of the substance operated on, and 

 Joule and Thomson's experiments have shown that this 

 scale differs but slightly from that of the ordinary air 

 thermometer. Joule had suggested to Thomson, in a 

 letter to him in 1848, that the probable value of Carnot's 

 function is the reciprocal of the absolute temperature as 

 measured on a perfect gas thermometer. 



Thus Clausius appears to have anticipated Thomson, 

 not in the suggestion of an absolute scale of temperature, 

 but in the idea of an absolute zero founded upon the 

 combination of Carnot's law and Joule's law. Thomson, 

 in his Papers, very modestly attributes the second law — 

 the law of the transformation of heat — to Carnot and 

 Clausius ; but in this he undervalued his work, because 

 Clausius appears to have assumed what Thomson and 

 Joule proved ; that is, the coincidence of the absolute scale 

 with the air thermometer scale. 



It will thus be seen that the position usuallv assumed 

 by the engineer at 1S50, of the equality between heat 

 givon to the engine and heat given to the' condenser, was 

 ■fundamentally untrue. Without this deduction, however, 



XO. 2030, VOL. 78] 



no determination of values of the Carnot function could 

 have led to the determination of an absolute zero. Accord- 

 ing to the material theory, as seen in the light of Carnot's 

 cycle, a heat unit could give an indefinitely increased 

 amount of work with lowering of the temperature. 

 Nothing in the theory sets a limit to this increase, and, 

 accordingly, there is nothing to suggest an absolute zero. 

 Immediately, however, we accept the dynamical theory of 

 heat we find that a pound of water requires the exertion 

 of 1390 foot-pounds of work to heat it through 1° C. We 

 also know from the Carnot cycle that under ordinary 

 conditions of human existence only a portion of this work 

 can be returned ; but as no conditions could conceivably 

 exist in which a greater amount of work could be obtained 

 from a pound of water than the 1390 foot-pounds put into 

 it to heat it through 1° C, it follows that, inasmuch as 

 the Carnot function increases with diminishing tempera- 

 ture, the limit of temperature is reached when, according 

 to the Carnot cycle, the whole of that work, put into the 

 pound of water, can be got out again as work. This 

 limit is the absolute zero of temperature. No lower 

 temperature is conceivable without introducing the idea 

 of the creation of energy. So far as human beings are 

 concerned, this idea is as inconceivable as the idea of the 

 creation of matter. The determination of this limit with 

 the close accuracy necessary for a well-founded constant 

 is to be entirely attributed to Thomson and Joule. In 

 his 1851 Paper Thomson thus succeeds in answering the 

 questions which he put to himself in his 1849 Paper, and 

 he supplies a quantitative method of connecting the amount 

 of the thermal agency necessary with the amount of work 

 which can be performed under varying conditions. 



Engineers dealing with motive power are thus deeply 

 in debt to Thomson and Joule for the secure position 

 occupied by them to-day. 



The brilliant work of Meyer, published so early as 1842, 

 is held by some to have anticipated to a large extent both 

 the work of Thomson and of Joule. Undoubtedly Meyer 

 formulated true ideas and carried his generalisations 

 through a wide range. Helmholtz also very early arrived 

 at similar conclusions to those of Joule and Thomson ; 

 but it has been thought better to discuss the work of 

 Thomson and Joule separately, in order to illustrate the 

 transition period through which many distinguished minds 

 W'ere passing about the time. Undoubtedly great credit is 

 due to Meyer, Helmholtz, Clausius, and Hirn, and 

 Thomson himself recognised this in the most generous 

 way. 



The ideas of Thomson and Joule now form so much 

 of the basis of all reasoning upon motive-power engines 

 that there is some little danger to the present generation 

 of forgetting what they owe to these two great men. To 

 appreciate the step made by them it is necessary to 

 consider the position of motive power produced by heat 

 at about the middle of the last century. At that time 

 many attempts had been made to displace the steam engine 

 as a heat engine by air engines in various forms — both 

 engines heated externally and those heated internally, now 

 known as internal-combustion engines. Papers read at 

 the Institution of Civil Engineers in 1845 and 1853, and 

 the discussion of those Papers by eminent men of the 

 day, supply an accurate measure of the knowledge possessed 

 by the engineer of the principles of action of his heat 

 engines. Many distinguished names occur in these Papers 

 and Discussions, including James Stirling, Robert Stephen- 

 son, Sir George Cayley, Charles Manby, James Leslie, 



C. W. Siemens, Hawksjey, Pole, W. G. Armstrong (after- 

 wards Lord Armstrong), Edward Woods, E. A. Cowper, 



D. K. Clark, Benjamin Cheverton, Goldsworthy Gurney, 

 George P. Bidder, Prof. Faraday. Isambard K. Brunei, 

 Captain Fitzroy, and F. Braithwaite. At the date of the 

 later of these discussions Brunei had already designed 

 the Great Eastern, in 1852, with its engines of 11,000 

 horse-power. .Armstrong was a Fellow of the Royal 

 Society, and had started the Elswick Works and invented 

 the .Armstrong gun. Robert Stephenson was at the height 

 of his fame. He was then a Member of Parliament, 

 President of the Institution of Civil Engineers, and a 

 Fellow of the Royal Society. Siemens was a young man, 

 but was busv on the .regenerative furnace : had considered 



