ON THE KINETIC THEORY OF GASES. 1 



IN the former of the two works which I have taken for 

 the subject of this article, Dr. Watson has given us a 

 clear account of the established results of the kinetic 

 theory. He has also pointed out the nature of some of the 

 difficulties of the theory and given valuable suggestions for 

 their solution. 



In Professor Tait's work, as yet unfinished, we have the 

 advantage of seeing the subject from a different point of 

 view. He has ventured a little farther than Dr. Watson 

 into the regions of hypothesis. He has also discussed the 

 difficult subjects of diffusion and viscosity in gases, with 

 which Dr. Watson, following his original plan, has not 

 attempted to deal. 



In discussing doubtful points I shall have to refer to the 

 arguments of these writers, and if this is done with some 

 abbreviation, I hope that the reader will supply the defects 

 of my exposition by reference to the originals. 



i. The simplest conception of a gas in the kinetic 

 theory is that of a number of perfectly elastic spheres 

 moving within an elastic bounding surface. If/ denote 

 pressure per unit of surface, v volume, T the mean kinetic 

 energy of a sphere, the equation pv = CT (A) is accurate, 

 provided that the spheres be mere particles, having mass 

 but no sensible diameters. 



2. The laws of Boyle and Charles, which hold approxi- 

 mately for hydrogen and other permanent gases under 

 ordinary conditions, are concisely expressed by the equa- 

 tion pv = G/ (B), in which t is the absolute temperature. 

 The complete agreement of (A) and (B) suggests the 

 kinetic theory, according- to which the molecules ol a gas 

 are to be regarded as elastic spheres, and the absolute 

 temperature is measured by their mean kinetic energy. 



1 A Treatise on the Kinetic Theory of Gases, by H. W. Watson, D.Sc, 

 F.R.S., second edition, Clarendon Press, Oxford, 1893. " On the Founda- 

 tion of the Kinetic Theory of Gases," parts i.-iv., by Professor Tait, Trans- 

 actions of the Royal Society of Edinburgh. 



