TRANSACTIONS OF SECTION A. 61 7 



to prove wholly futile ? But now, instead of imagining the question : What do 

 you mean by explaining a property of matter? to be put cynically, and letting 

 ourselves be irritated by it, suppose we give to the questioner credit for being 

 sympathetic, and condescend to try and answer his question. "We find it not very 

 easy to do so. All the properties of matter are so connected that we can scarcely 

 imagine one thoroughly explained without our seeing its relation to all the others, 

 without in fact Laving the explanation of all; and till we have this we cannot tell 

 what we mean by ' explaining a property,' or ' explaining the properties ' of matter. 

 But though this consummation may never be reached by man, the progress of 

 science may be, I believe will be, step by step towards it, on many different roads 

 converging towards it from all sides. The kinetic theory of gases is, as I have said, 

 a true step on one of the roads. On the very distinct road of chemical science, 

 St. Clair Deville arrived at his grand theory of dissociation without the slightest 

 aid from the kinetic theory of gases. The fact that he worked it out solely from 

 chemical observation and experiment, and expounded it to the world without any 

 hypothesis whatever, and seemingly even without consciousness of the beautiful 

 explanation it has in the kinetic theory of gases, secured for it immediately an 

 independent solidity and importance as a chemical theory when he first promul- 

 gated it, to which it might even by this time scarcely have attained if it had first 

 been suggested as a probability indicated by the kinetic theory of gases, and been 

 only afterwards confirmed by observation. Now, however, guided by the views 

 which Olausius and Williamson have given us of the continuous interchange of 

 partners between the compound molecules constituting chemical compounds in the 

 gaseous state, we see in Deville's theory of dissociation a point of contact of the 

 most transcendent interest between the chemical and physical lines of scientific 

 progress. 



To return to elasticity : if we could make out of matter devoid of elasticity a 

 combined system of relatively moving parts which, in virtue of motion, has the 

 essential characteristics of an elastic body, this would surely be, if not positively 

 a step in the kinetic theory of matter, at least a finger-post pointing a way which 

 we may hope will lead to a kinetic theory of* matter. Now this, as I have already 

 shown, ' we can do in several ways. In the case of the last of the communications 

 referred to, of which only the title has hitherto been published, I showed that, from 

 the mathematical investigation of a gyrostatically dominated combination contained 

 in fhe passage of Thomson and Tait's ' Natural Philosophy ' referred to, it follows 

 that any ideal system of material particles, acting on one another mutually through 

 massless connecting springs, may be perfectly imitated in a model consisting of 

 rigid links jointed together, and having rapidly rotating fly-wheels pivoted on 

 some or on all of the links. The imitation is not confined to cases of equilibrium. 

 It holds also for vibration produced by disturbing the system infinitesimally from a 

 position of stable equilibrium and leaving it to itself. Thus we may make a 

 gyrostatic system such that it is in equilibrium under the influence of certain 

 positive forces applied to different points of this system ; all the forces being 

 precisely the same as, and the points of application similarly situated to, those of the 

 stable system with springs. Then, provided proper masses (that is to say, proper 

 amounts and distributions of inertia) be attributed to the links, we may remove 

 the external forces from each system, and the consequent vibration of the points 

 of application of the forces will be identical. Or we may act upon the systems 

 of material points and springs with any given forces for any given time, and leave 

 it to itself, and do the same thing for the gyrostatic system ; the consequent 

 motion will be the same in the two cases. If in the one case the springs are 

 made more and more stiff, and in the other case the angular velocities of the fly r - 

 wheels are made greater and greater, the periods of the vibrational constituents of 



1 Paper on 'Vortex Atoms,' Proc. R. S. E. Feb. 1807 ; abstract of Lecture before 

 Royal Institution of Great Britain, March 4, 1881, on ' Elasticity viewed as possibly 

 a Mode of Motion ; ' Thomson and Tail's Natural Philosophy, second edition, Part I. 

 §§ 345 vjjj to 315 xxvjj ; ' On Oscillation and Waves in an Adynamic Gyrostatic 

 System' (title only), Proc. R. S. E. March 1883. 



