104 Dr. W. J. M. Rankine on Thermodynamics. 



minate ; and work done in altering the capacity or the figure of 

 the vessel does not produce heat, but only stored up energy, like 

 that possessed by a bent spring. 



A system of particles confined within a vessel, and not sen- 

 sibly attracting or repelling each other, but in a state of motion, 

 exerts outward pressure against the internal surface of that vessel 

 through the reactions of the particles that tend to escape, but 

 are prevented by the vessel from doing so ; and work done in 

 diminishing the capacity of this vessel wholly takes effect in ac- 

 celerating the motions of the confined particles, — that is, in the 

 language of thermodynamics, producing heat. 



The condition of actual bodies is compounded of those two ; 

 and it is by means of an equation deduced from what has been 

 called the " Second Law " of thermodynamics, that the force ex- 

 erted by a substance against the internal surface of a vessel con- 

 taining it (in other words, the elastic force of the substance) is 

 distinguished into two components, due respectively to mole- 

 cular attractions and repulsions, and to the reactions of moving 

 particles (of the nature of centrifugal force). It is the latter 

 component of the force only that is taken into account in calcu- 

 lating how much heat is produced by a given alteration of the 

 dimensions or figure of the containing vessel. 



It has been proved by experiment that very nearly the whole 

 of the work done in compressing a gas takes effect in producing 

 heat; and hence it has been concluded that the elasticity of gases 

 is almost wholly due to the motion of their particles, the compo- 

 nent due to attractions and repulsions being small in comparison. 



The detailed exposition of the principles to which I have briefly 

 referred, and the comparison of their results with those of expe- 

 riment, have been made so often, by so many authors, and in so 

 many ways, that it would be a waste of time and space for me to 

 explain them further here ; and I shall therefore, in conclusion, 

 merely refer to Professor Tait's work on Thermodynamics as the 

 best source of information regarding the history and present 

 condition of that science; for he gives a summary, in very mo- 

 derate compass, of the different methods of demonstration fol- 

 lowed by the various original authors. In most of the popular 

 writings on the subject, the second law of thermodynamics, 

 together with its proofs and consequences, is omitted, as requi- 

 ring too much mental exertion for its comprehension. 

 I am, Gentlemen, 



Your most obedient Servant, 



W. J. Macquorn Rankine. 

 Glasgow, July 5, 18/0. 



