August 25, 192 1] 



NATURE 



805 



a new edition is required. By itself this indicates 

 the value of the work, and it is also very satis- 

 factory as showing the growth of interest in this 

 important branch of mathematical physics. The 

 main part of the book has scarcely been altered^ — 

 indeed, too little so, for a good many of the mis- 

 prints of the second edition have appeared again. 

 In dealing with viscosity and heat conduction, 

 Chapman's important work is given somewhat 

 more fully than before ; but it would, of course, 

 be out of the question to reproduce in detail the 

 stupendous formulae which it involves. The only 

 other point we will mention is to express a doubt 

 whether the author's explanation of irreversibility 

 really does explain that exceptionally difficult 

 question. To put it in an extreme form, has any- 

 one yet really discovered what distinguishes the 

 past from the future? 



The main changes are the additions at the end 

 of the book, whjch deal with the quantum theory, 

 and it must be confessed that we found these in 

 some ways rather disappointing. A certain lack 

 of harmony is produced by the policy of grafting 

 a few chapters on the quantum on to the end of 

 a book originally written before its existence was 

 accepted. It must be recognised that it is a very 

 difficult task to fit the subject in, for it is clearly 

 right to give a complete account of the quantum ; 

 yet its field is a great deal wider than the mere 

 kinetic theory of gases, so that its introduction 

 necessitates the treatment of several other 

 branches of physics, some of which are by no 

 means elementary. What the author has given is 

 certainly the most important part of the quantum 

 theory — there is an excellent account of spectra, 

 and also of the Debye theory of the specific heats 

 of solids. The theories of Tetrode, Keesom, and 

 others on the equations of state of gases, how- 

 ever, are barely mentioned ; it is true they rest on 

 much less firm foundations than the other ques- 

 tions, but still they are far more closely related 

 to the subject-matter of the rest of the book, and 

 their exposition would not have taken very long. 

 Also the author gives only a very short discussion 

 of the rotations of molecules, though there is 

 direct experimental evidence as to their moments 

 of inertia, and though Ehrenfest's formula for the 

 specific heat of a gas is a type of function novel to 

 the quantum theory. Again, it would have been in- 

 teresting to have had more of an exposition of the 

 method of solving problems by direct use of the 

 relation of entropy to probability as typified by 

 Planck's original calculation of the radiation 

 formula. This method seems to us, on the whole, 

 inferior to the author's, but it has been used a 

 great deal, and must be understood by anyone who 

 wishes to read the original papers of the subject. 

 NO. 2704, VOL. 107] 



In view of the greatly extended field that these 

 chapters cover, only shortened proofs of many 

 important theorems are given, and some of these 

 are not fortunate. For example, in dealing with 

 the displacement, law of Wien the author states 

 that the energy in each wave-length is unaltered 

 during the change of wave-lengths, whereas in fact 

 part of this energy is turned into work, and its 

 disappearance is the essence of the process. Again, 

 the author calculates the equilibrium between the 

 energy of a vibrator and that of the surrounding 

 electromagnetic field by finding separately the 

 amounts of energy absorbed and emitted. This 

 is essentially a problem of resonance, but in the 

 calculation of the absorption the damping factor 

 is omitted without justification. In this particular 

 case the correct proof is no longer or more diffi- 

 cult than the author's. 



In a subject like this, based as it is on very 

 uncertain foundations, it must have been excep- 

 tionally hard to select what was sufficiently 

 well established to merit inclusion. It will 

 be seen that our chief quarrel with the author is 

 that he did not give us enough. The book con- 

 tains a great deal of invaluable information critic- 

 ally treated, which it would be hard to find else' 

 where in English. If we have laid emphasis on 

 the defects, it is because the excellences of the 

 work are well known. C. G. D. 



Beast and Man in India. 



Companions : Feathered^ Furred, and Scaled. By 

 C. H. Donald. Pp. ix-t-159. (London: John 

 Lane; New York: John Lane Co., 1920.) 

 7s. net. 



THESE are vivaciously written reminiscences of 

 Indian animals with which Mr. Donald man- 

 aged to establish friendly relations. The first is the 

 tale of a bear-cub, Bhaloo, with a strong sense 

 of humour which became very expensive to his 

 owner. The second tells of the rearing of two 

 weaver-birds {Ploceus baya), which justified their 

 reputation for inquisitiveness and educability. A 

 weaver, "carefully and kindly taught, will, within 

 a week, let off a toy cannon, select a particular 

 number out of many cards, and bring it to his 

 master ; he will catch a two-anna piece which has 

 been thrown into a well before it reaches the 

 water, and bring it back. Some of his tricks 

 seem absolutely incredible, and yet they are sim- 

 plicity itself, and one and all may be taught in 

 a couple of days each. The first and most im- 

 portant step in his training is to teach him that 

 an open hand means food, and that a closed fist 

 does not. Everything hinges on his mastery of 



