REVIEWS ;oi 



standing on the part of the beginner ; for that reason might this work be put in the 

 hands of any intelligent student, if only to read the earlier pages which do not deal 

 with the author's special hypothesis. 



But of course it is the author's development of the " Quantum of Action " 

 theory which constitutes the most interesting feature of the volume. That 

 theory, although imperfectly developed at present, has nevertheless provided a 

 formula for the density of radiation under given conditions which has been applied 

 with signal success to measurements on specific heats and the photo-electric effect, 

 and to the calculation of the natural unit of electricity. That a physical ''model" 

 of the theory is still wanting, that on questions as to the "atomicity " or "con- 

 tinuity" of energy there should be diverse opinions among the experts, is only to 

 be expected in view of the recent introduction of the hypothesis and the far- 

 reaching consequences of its adoption. Not the least of these consequences will 

 be the abandonment (except as a good working summary of average effects) of the 

 laws of mechanics and electrodynamics as hitherto laid down. At bottom Planck's 

 views constitute the first attempt, promising success, which will take account in 

 our fundamental equations of that irreversibility which is such a marked char- 

 acteristic of all natural phenomena, and whose existence would be unsuspected by 

 any one guided solely by our classical mechanics and electrodynamics. 



The author is well aware of the present imperfections of his theory, and, in a 

 conservative endeavour to mitigate somewhat the blow dealt at established views, 

 he has, in this second edition of his book, receded somewhat from the position 

 which he occupied in his earlier papers and in the 1906 edition of the present 

 work. He now succeeds in obtaining his well-known radiation formula by 

 assuming that the atomic and subatomic mechanisms which are responsible for 

 exchange of energy by radiation, emit discontinuously or in "lumps,' but absorb 

 continuously ; this is in contradistinction to his earlier views, which postulated both 

 discontinuous absorption and emission. He still maintains that his hypothesis, 

 whether in the earlier or more recent form, does not commit one to an assumption 

 of an atomic structure of energy itself. The newer presentation does to some 

 extent meet certain objections launched at the older, and the author believes it 

 to be less " revolutionary." This may well be doubted ; at all events it is more 

 abstruse, and rests, even more than his first exposition, on a new view concerning 

 general theorems in probability, which may prove vague and unsatisfying to 

 people little acquainted with the modern methods of that most "slippery" of 

 subjects. 



The translation is well done, and the translator has added a useful bibliography 

 of important papers and a demonstration of certain mathematical formulae used 

 in the text, but not likely to be familiar to the general reader. 



J. R. 



The Electron Theory of Matter. By O. W. Richardson, M.A., D.Sc, F.R.S. 

 [Pp. vi + 612.] (London: Cambridge University Press, 1914. Price 

 iSs. net.) 



This century has already produced considerable alterations in the centre of 

 interest of many branches of science ; in none more than in Electricity. Twenty 

 year ago the " electron " was experimentally unknown ; it had made its appearance 

 as a postulate in certain papers of Lorentz dealing with the mathematical theory 

 of dispersion, etc. The genius of Faraday and Maxwell had impressed on student 

 and teacher alike the doctrine that in the medium alone were the essential 

 46 



