﻿62 Dr. W. J. M. Rankine on the Hypothesis of 



limited. It would still remain certain that the zodiacal light is 

 not a terrestrial appendage (either near or far off); but what sort 

 of solar appendage it might be would be a problem as difficult 

 to solve as that presented by comets. 



If the radiated structure of the sun's corona as seen under 

 favourable atmospheric conditions should be confirmed as more 

 than an optical phenomenon, it is not impossible that we might 

 be put in the way of interpreting the zodiacal light. 



X. On the Hypothesis of Molecular Motions in Thermodynamics. 

 By W.J. Macquorn Rankine, C.E.,LL.D., F.R.SS.Lond. 



fyEdin* 



1. TF I rightly understand the question raised by Mr. Heath, 

 J- it is not whether the two propositions called respectively 

 the First Law and the Second Law of Thermodynamics correctly 

 express the facts of experiments respecting the relations between 

 heat and mechanical energy from which those laws have been 

 arrived at by induction, but whether those two laws follow from 

 the hypothesis that thermometric heat consists in some sort of 

 molecular motion — and in particular whether the contraction of 

 the space within which a system of moving bodies is confined 

 causes of itself acceleration of their motion ; for if this be esta- 

 blished, the two laws of thermodynamics readily follow from 

 the hypothesis. 



2. According to a proposition laid down by all the writers 

 on thermodynamics who have treated of the before- mentioned 

 hypothesis, the acceleration of the motion of a system of bodies 

 by the contraction of the space within which they are confined 

 is a necessary consequence of the elementary principles of dyna- 

 mics. According to Mr. Heath's views, unless I misunderstand 

 them, that proposition is inconsistent with the principle that a 

 body under the action of a pair of equal and opposite forces is 

 neither accelerated nor retarded — in other words, with the first 

 law of motion. 



3. In order that the alleged inconsistency should exist, it 

 would be necessary that each of the bodies under consideration 

 should be under the action of no force, or of balanced forces. 

 But that is not so ; for the bodies under consideration are par- 

 ticles moving within a limited space ; therefore their motions in 

 any given direction are limited in extent ; therefore during cer- 

 tain parts of their motions, and especially when they are near 

 the boundaries of the containing space, those motions must be 

 continually varying, certainly in direction, and possibly both in 



* Communicated by the Author. 



