258 



NATURE 



\Jan. 31, 1878 



set himself to imagine the mechanism by which they 

 might be produced. Being an accomplished engineer, he 

 succeeded in specifying a particular arrangement of 

 mechanism competent to do the work, and also in 

 predicting other properties of the mechanism which 

 were afterwards found to be consistent with observed 

 facts. 



As long as the training of the naturalist enables him to 

 trace the action only of particular material systems 

 without giving him the power of dealing with the general 

 properties of all such systems, he must proceed by the 

 method so often described in histories of science — he 

 must imagine model after model of hypothetical apparatus 

 till he finds one which will do the required work. If this 

 apparatus should afterwards be found capable of account- 

 ing for many of the known phenomena, and not demon- 

 strably inconsistent with any of them, he is strongly 

 tempted to conclude that his hypothesis is a fact, at least 

 until an equally good rival hypothesis has been invented. 

 Thus Rankine,' long after an explanation of the properties 

 of gases had been founded on the theory of the collisions 

 of molecules, published what he supposed to be a proof 

 that the phenomena of heat were invariably due to steady 

 closed streams of continuous fluid matter. 



The scientific career of Rankine was marked by the 

 gradual development of a singular power of bringing the 

 most difficult investigations within the range of elemen- 

 tary methods. In his earlier papers, indeed, he appears 

 as if battling with chaos, as he swims, or sinks, or wades, 

 or creeps, or flies, 



" And through the palpable obscure finds out 

 His uncouth way j " 



but he soon begins to pave a broad and beaten way over 

 the dark abyss, and his latest writings show such a power 

 of bridging over the difficulties of science, that his pre- 

 mature death must have been almost as great a loss 

 to the diff'usion of science as it was to its advancement. 



The chapter on thermodynamics in his book on the 

 steam-engine was the first published treatise on the 

 subject, and is the only expression of his views addressed 

 directly to students. 



In this book he has disencumbered himself to a great 

 extent of the hypothesis of molecular vortices, and builds 

 principally on observed facts, though he, in common with 

 Clausius, makes several assumptions, some expressed as 

 axioms, others implied in definitions, which seem to us 

 anything but self-evident. As an example of Rankine's 

 best style we may take the following definition : — 



" A PERFECT GAS is a substance in such a condition 

 that the total pressure exerted by any number of portions 

 of it, at a given temperature, against the sides of a vessel 

 in which they are inclosed, is the sum of the pressures 

 v/hich each portion would exert if inclosed in the vessel 

 separately at the same temperature." 



Here we can form a distinct conception of every clause 

 of the definition, but when we come to Rankine's Second 

 Law of Thermodynamics we find that though, as to 

 literary form, it seems cast in the same mould, its actual 

 meaning is inscrutable. 



" The Second Law of Thermodynamics. — If the total 



' On ihe Second Law of Thermodynamics. Phil. Mag. Oct. 1865, § 12, 

 p 244 ; butinhispaperon the Thermal Energy of Molecular Vortices, Trans. 

 R S, Kdin , xxv. p. 557 [1869] he admits that the explanation of gaseous 

 pressure by the impacts of molecules has been proved to be possible. 



actual heat of a homogeneous and uniformly hot sub- 

 stance be conceived to be divided into any number of 

 equal parts, the effects of those parts in causing work to 

 be performed are equal." 



We find it difficult enough, even in 1878, to attach any 

 distinct meaning to the total actual heat of a body, and 

 still more to conceive this heat divided into equal parts, 

 and to study the action of each of these parts, but as if 

 our powers of deglutition were not yet sufficiently strained, 

 Rankine follows this up with another statement of the 

 same law, in which we have to assert our intuitive belief 

 that— 



" If the absolute temperature of any uniformly hot 

 substance be divided into any number of equal parts, the 

 effects of those parts in causing work to be performed are 

 equal." 



The student who thinks that he can form any idea of 

 the meaning of this sentence is quite capable of explaining 

 on thermodynamical principles what Mr. Tennyson says 

 of the great Duke : — 



" Whose eighty winters freeze with one rebuke 

 All great self-seekers trampling on the right." 



Prof. Clausius does not ask us to believe quite so much 

 about the heat in hot bodies. In his first memoir, indeed, 

 he boldly dismisses one supposed variety of heat from the 

 science. Latent heat, he tells us, "is not only, as its 

 name imports, hidden from our perceptions, but has 

 actually no existence ; " " it has been converted into 

 work." 



But though Clausius thus gets rid of all the heat which, 

 after entering a body, is expended in doing work, either 

 exterior or interior, he allows a certain quantity to remain 

 in the body as heat, and this remnant of what should 

 have been utterly destroyed lives on in a sort of smoulder- 

 ing existence, breaking out now and then with just enough 

 vigour to mar the scientific coherence of what might have 

 been a well compacted system of thermodynamics. 



Prof. Tait tells us :— 



" The source of all this sort of speculation, which is as 

 old as the time of Crawford and Irvine — and which was 

 countenanced to a certain extent even by Rankine — is the 

 assumption that bodies must contain a certain quantity of 

 actual, or thermometric, heat. We are quite ignorant 

 of the condition of energy in bodies generally. We know 

 how much goes in, and how much comes out, and we 

 know whether at entrance or exit it is in the form of heat 

 or of work. But that is all." 



If we define thermodynamics, as I think we may now 

 do, as the investigation of the dynamical and thermal 

 properties of bodies, deduced entirely from what are called 

 the First and Second laws of Thermodynamics, without 

 any hypotheses as to the molecular constitution of bodies, 

 all speculations as to how much of the energy in a body 

 is in the form of heat are quite out of place. 



Prof. Tait, however, does not seem to have noticed that 

 Prof. Clausius, in a footnote to his sixth memoir,^ tells us 

 what he means by the heat in a body. In the middle of a 

 sentence we read : — 



" .... the heat actually present in a unit weight of 

 the substance in question— in other words, the vis viva 

 of its molecular motions " . . . . 



Thus the doctrine that heat consists of the vis viva o 



' Hirst's translation, p. 230, Germ.nn edition, 1864, p. 258, "wirklich 

 vorhandcne Warme, d.h. die lebendige Kraft seiner Molecularbewegimgen. 



