11 .REPORT 1865. 



law would obviously follow from the supposition of any kind of molecular motion 

 whatsoever, and it therefore affords of itself no reason for preferring one supposi- 

 tion as to the kind of molecular motion which constitutes sensible heat to another. 

 But if there be molecular motions in bodies, it is certain that, although all of them 

 are capable of conversion into that which constitutes sensible heat, some of them 

 are not accompanied by sensible heat. For example, the motion, supposed to be 

 vibratory and wave-like, which constitutes radiance, whether visible or invisible, 

 is not accompanied by sensible heat, and only produces sensible heat by its absorp- 

 tion, that is, in the language of hypothesis, by its conversion into some other kind 

 of motion ; while, on the other hand, in the production of radiance sensible heat 

 disappears. The object of the paper, then, is to give an elementary proof of the 

 proposition (of which the author gave an intricate algebraical demonstration in the 

 Transactions of the Royal Society of Edinburgh for 1851), that the second law of 

 thermodynamics follows from the supposition that sensible heat consists in any kind 

 of steady molecular motion within limited spaces. Steady motion may be defined 

 as motion in a set of streams of invariable figure. When steady motion takes place 

 in matter that is confined within a limited space, the streams in which the particles 

 move must necessarily return into themselves, and be circulating streams, being, in ' 

 that respect, of the nature of whirls or vortices. Steady motion keeps unaltered 

 the distribution of the density of the moving matter; and it therefore keeps unaltered 

 the forces depending on such distribution whether of the nature of pressure or of 

 attraction. In this respect it differs from unsteady motion, such as vibratory and 

 wave-like motion. Supposing that the dimensions of the limited space in which 

 the moving matter is enclosed undergo an indefinitely small change by the appli- 

 cation of suitable forces, and that after that process the motion becomes steady as 

 it was before, then the dimensions and position of each circulating stream will have 

 been altered, and the work done in effecting that alteration will consist of energy 

 converted between the forms of potential energy of the applied forces and actual 

 energy of the molecular motions — that is, between the forms of mechanical energy 

 and heat. Prof. Rankine then works out the problem to this conclusion— that, if 

 sensible heat consists in any land of steady molecular motion within limited spaces, 

 the conversion of energy during any change in the dimensions of such spaces is the 

 product of the .absolute temperature into some function of that change and of the 

 sort and distribution of the matter. It is obvious that the steadiness of the sup- 

 posed molecular motion is the essential condition which makes the second law of 

 thermodynamics deducible from a mechanical hypothesis, and that no kind of un- 

 steady motion, such as vibratory or wave-like motion, would lead to the same 

 results. If then it be admitted as probable that the phenomena of heat are due to 

 unseen molecular motions, it must also be admitted that, while the motions which 

 constitute radiance are vibratory and wave-like, the motions which constitute sen- 

 sible or thermometric heat must be steady, and like those of circulating streams. 

 The function by which the absolute temperature is multiplied in calculating the 

 conversion of energy between the mechanical and the thermic forms is the variation 

 of what the author has called the "metamorphic function," being one term of the 

 differential of the "thermodynamic function," which corresponds to what Prof. 

 Clausius calls " entropy." 



Electricity. 



On India -Ruhhcr and Gulta Pereha as Insulators for Submarine Telegraphic 

 Cables. By W. Fairbairn, F.B.S. 



Mr. Fairbairn stated as his opinion that gutta pereha, although not so perfect an 

 insulator as india-rubber, was nevertheless preferable for deep-sea cables subjected 

 to great pressure. He found by experiment' that the conductivity of the core was 

 improved by gutta-percha insulation under severe pressure to a greater extent 

 than by india-rubber or any other insulating material ; and he believed it was more 

 durable and better calculated to resist the action of salt water at great depths. 



