204 



S'CtENCE. 



[Vol. IV., No. 83. 



science than this. The committees are gen 

 erally continued from year to year, and thus 

 form permanent working bodies, each pursuing 

 a definite object. Naturally, nearly all the 

 work will be done by one or a small number of 

 members ; but the latter have the advantage 

 and stimulus of the co-operation and advice 

 of their fellow members, who again are in a 

 position to provide for the continuance of the 

 work in case the person in charge gives it up. 

 An idea of the present importance of this fea- 

 ture of the organization may be gained from 

 the fact, that there are more than forty such 

 committees at work, reporting annually. Some 

 examples may be cited to show the character 

 of the work undertaken : A committee on 

 underground temperature collects determina- 

 tions of the rate of increase of temperature in 

 mines, and other places where it is possible to 

 determine it ; another is collecting and inves- 

 tigating meteoric dust ; another investigates 

 the lunar disturbance of gravity ; Mr. Francis 

 Galton is at the head of a committee devising 

 a system of statistical measurements of human 

 beings ; the economists have a committee in- 

 vestigating the rate of wages, and its relation 

 to economic progress. In a word, a range of 

 subjects from tables of binar}^ quantics to pat- 

 ent legislation, and the migration of birds, are 

 being regularly investigated. We wish there 

 could be a body of men in this country pursu- 

 ing similar objects. 



The number of Americans present at the 

 meeting was even greater than could have been 

 expected ; and the high character of the Ameri- 

 can representation is sufficiently shown by the 

 fact that six ex-presidents of the American 

 association were in attendance. 



STEPS TOWARDS A KINETIC THEORY 

 OF MATTER. 1 



The now well-known kinetic theory of gases is a 

 step so important, in the way of explaining seemingly 

 static properties of matter by motion, that it is scarcely 

 possible to help anticipating, in idea, the arrival at a 

 complete theory of matter, in which all its properties 

 will be seen to be merely attributes of motion. 



Rich as it is in practical results, the kinetic theory 

 of gases, as hitherto developed, stops absolutely short 

 at the atom or molecule, and gives not even a sug- 

 gestion towards explaining the properties in virtue 

 of which the atoms or molecules mutually influence 

 one another. 



1 Address to the mathematical and physical section of the 

 British association at Montreal, Aug. 28, 1884, by Professor Sir 

 William Thomson, M.A., LL.D., D.C.L., F.R.S., L. & E., 

 F.R.A.S., president of the section. 



Every one who has hitherto written or done any 

 thing very explicit in the kinetic theory of gases has 

 taken the mutual action of molecules in collision as 

 repulsive. May it not, after all, be attractive ? Im- 

 agine a great multitude of particles enclosed by a 

 boundary which may be pushed inwards in any part, 

 all round, at pleasure. Now station an engineer 

 corps of Maxwell's army of sorting demons all round 

 the enclosure, with orders to push in the boundary 

 diligently everywhere when none of the besieged 

 troops are near, and to do nothing when any of them 

 are seen approaching, and until after they have 

 turned again inwards. The result will be, that, with 

 exactly the same sum of kinetic and potential ener- 

 gies of the same enclosed multitude of particles, the 

 throng has been caused to be denser. Now, Joule's 

 and Thomson's old experiments on the efflux of air 

 prove, that if the crowd be common air, or oxygen, 

 or nitrogen, or carbonic acid, the temperature is a 

 little higher in the denser than in the rarer condition 

 when the energies are the same. By the hypothesis, 

 equality of temperature between two different gases, 

 or two portions of the same gas at different densities, 

 means equality of kinetic energies in the same num- 

 ber of molecules of the two. From the observations 

 proving the temperature to be higher, it therefore 

 follows that the potential energy is smaller in the 

 condensed crowd. This (always, however, under 

 protest as to the temperature hypothesis) proves 

 some degree of attraction among the molecules, but 

 it does not prove ultimate attraction between two 

 molecules in collision, or at distances much less than 

 the average mutual distance of nearest neighbors in 

 the multitude. 



We must look distinctly on each molecule as being 

 either a little elastic solid, or a configuration of mo- 

 tion in a continuous, all-pervading liquid. How we 

 can ever permanently rest anywhere short of this last 

 view is not evident ; but it would be a very pleasant 

 temporary resting-place on the way to it, if we could, 

 as it were, make a mechanical model of a gas out of 

 little pieces of round, perfectly elastic, solid matter, 

 flying about through the space occupied by the gas, 

 and colliding with one another, and against the sides 

 of the containing vessel. But alas for a mechani- 

 cal model consisting of the cloud of little elastic 

 solids flying about amongst one another! Though 

 each particle have absolutely perfect elasticity, the 

 end must be pretty much the same as if it were but 

 imperfectly elastic. The average effect of repeated 

 and repeated mutual collisions must be to gradually 

 convert all the translational energy into energy of 

 shriller and shriller vibrations of the molecule. Even 

 if this fatal fault in the theory did not exist, and if 

 we could be perfectly satisfied with the kinetic the- 

 ory of gases founded on the collisions of elastic solid 

 molecules, there would still be beyond it a grander 

 theory, which need not be considered a chimerical 

 object of scientific ambition, — to explain the elas- 

 ticity of solids. 



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 



