PREFACE. 



TN writing the present book, my first aim has been to develop the Theory 

 -*- of Gases upon as exact a mathematical basis as possible. The need for 

 a sure foundation and an exact treatment is, I think, sufficiently obvious. 

 For some years the Kinetic Theory has been regarded by the great majority 

 of physicists with ever-increasing distrust and suspicion.'/ The origin of this 

 attitude of mind is undoubtedly to be looked for in the fact that many 

 results which have been deduced from the Theory are found to be in obvious 

 opposition to experimental results. Deductions which have any relation at 

 ' all to physical phenomena cannot of course be arrived at without definite 

 physical assumptions, and any inconsistency between theoretical and experi- 

 mental results must of necessity be traceable to imperfections in the physical 

 assumptions on which the development of the theory is based. Unfortunately 

 the assumptions frequently enter in so subtle a form that it is hardly possible 

 to realise that a definite assumption has been made at all. Hence the 

 imperative necessity for a strict logical treatment. I have hoped that some 

 of the distrust which at present attaches itself to the Kinetic Theory, will 

 be removed when the exact relation between premises and conclusions is 

 pointed out. 



One discrepancy between theory and experiment, that connected with the 

 theorem of Equipartition of Energy and the consequent evaluation of the 

 ratio of the specific heats of a gas, is of greater importance than all the 

 others together. As regards this particular question, I have tried to 

 emphasise the facts, (i) that the theorem of Equipartition is based upon a 

 definite assumption, namely that there is no interaction between matter 

 and ether, (ii) that wherever we look, in the most literal sense of the word, 

 the evidence that the assumption is illegitimate thrusts itself upon us every 

 ray of light which reaches our eyes is evidence against the truth of the 

 assumption and (iii) that as soon as we abandon the assumption in question, 

 theory and experiment harmonise as well as could be desired. 



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