TAITS "THERMODYNAMICS. 663 



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 elementary 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 ;" 



but he soon begins to pave a broad and beaten way over the dark abyss, and 

 his latest writings shew such a power of bridging over the difficulties of science, 

 that his premature death must have been almost as great a loss to the diffusion 

 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 enclosed, is the sum of the pressures which each portion would exert if enclosed 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. 



"On the Second Law of Thermodynamics," Phil. Mag. Oct. 1865, 12, p. 244; but in his 

 paper on the Thermal Energy of Molecular Vortices, Trans. E.S. Edin. xxv. p. 657 [1869], he 

 admits that the explanation of gaseous pressure by the impacts of molecules has been proved to 

 be possible. 



