XXIV PREFACE. 



reductions made in the course of his work discloses however an inaccuracy which materially 

 affects the values of the coefficient of viscosity obtained. Subsequent experiments also seem 

 to shew that the concise relation he endeavoured to establish is by no means so near 

 the truth as he supposed, and it is more than doubtful whether the action between two 

 molecules can be represented by any law of so simple a character. 



In the same memoir he gives a fresh demonstration of the law of distribution of 

 velocities, but though the method is of permanent value, it labours under the defect of 

 assuming that the distribution of velocities in the neighbourhood of a point is the same 

 in every direction, whatever actions may be taking place within the gas. This flaw in 

 the argument, first pointed out by Boltzmann, seems to have been recognised by Maxwell, 

 who in his next paper " On the Stresses in Rarefied Gases arising from inequalities of 

 Temperature," published in the Philosophical Transactions for 1879, Part I., adopts a form 

 of the distribution function of a somewhat different shape. The object of this paper was 

 to arrive at a theory of the effects observed in Crookes's Radiometer. The results of the 

 investigation are stated by Maxwell in the introduction to the paper, from which it would 

 appear that the observed motion cannot be explained on the Dynamical Theory, unless it 

 be supposed that the gas in contact with a solid can slide along the surface with a finite 

 velocity between places whose temperatures are different. In an appendix to the paper 

 he shews that on certain assumptions regarding the nature of the contact of the solid 

 and gas, there will be, when the pressure is constant, a flow of gas along the surface 

 from the colder to the hotter parts. The last of his longer papers on this subject is 

 one on Boltzmann's Theorem. Throughout these volumes will be found numerous shorter 

 essays on kindred subjects, published chiefly in Nature and in the Encyclopedia Britaimica. 

 Some of these contain more or less popular expositions of this subject which Maxwell 

 had himself in great part created, while others deal with the work of other writers in 

 the same field. They are profoundly suggestive in almost every page, and abound in acute 

 criticisms of speculations which he could not accept. They are always interesting; for 

 although the larger papers are sometimes difficult to follow, Maxwell's more popular writings 

 are characterized by extreme lucidity and simplicity of style. 



The first of Maxwell's papers on Colour Perception is taken from the Transactions of 

 the Royal Scottish Society of Arts and is in the form of a letter to Dr G. Wilson dated 

 Jan. 4, 1855. It was followed directly afterwards by a communication to the Royal Society 

 of Edinburgh, and the subject occupied his attention for some years. The most important 

 of his subsequent work is to be found in the papers entitled "An account of Experiments 

 on the Perception of Colour" published in the Philosophical Magazine, Vol. xiv. and "On 

 the Theory of Compound Colours and its relation to the colours of the spectrum " in the 

 Philosophical Transactions for the year 1860. We may also refer to two lectures delivered 

 at the Royal Institution, in which he recapitulates and enforces his main positions in hi- 

 usual luminous style. Maxwell from the first adopts Young's Theory of Colour Sensation, 

 according to which all colours may ultimately be reduced to three, a red, a green and 



