694 



NATURE 



[February 27, 19 13 



MODERN PHYSICS. 



(i) Studies in Radioactivity. By Prof. W. H. 

 Bragg, F.R.S. Pp. xi+196. Macmillan's 

 Science Monographs. (London : Macmillan 

 and Co., Ltd., 1912.) Price 5s. net. 



(2) The Electrical Properties of Flames and of 

 Incandescent Solids. By Prof. H. A. Wilson, 

 F.R.S. Pp. vii+119 (London: University 

 of London Press; Hodder and Stoughton,i9i2.) 

 Price 6s. net. 



IT is seldom in the history of any science 

 that three fundamental discoveries are 

 included within the brief space of three years. 

 The discovery of X-rays in 1895 marks a new 

 epoch in the history of physical science, for it 

 led early in 1896 to the discovery of radio-activity 

 and was followed in 1897 by the proof of the 

 nature of the kathode rays and the advent of the 

 electron as a definite entity. In the following 

 years an ever-increasing fraction of the energy of 

 workers in physics has been devoted to a study of 

 the numerous important problems which have 

 arisen from these three primary discoveries. 



In the early stages of the experimental inquiry 

 a discussion of these subjects was conveniently 

 included in single treatises on the conduction of 

 electricity through gases and on radio-activit} . 

 With the rapid increase of our knowledge of the 

 various subdivisions of the subject it was in- 

 evitable that special treatises would be necessary 

 to discuss in more detail the results of recent 

 investigations. This is illustrated by the separate 

 publications that have appeared on the theory of 

 electrons, ionisation b}' collision, and the chemistry 

 of the radio-active bodies. This tendency towards 

 specialisation is in many respects advantageous 

 provided two conditions are fulfilled. In the first 

 place, it is essential that the subject should be 

 treated by experts who have taken an active part 

 in the development of our knowledge of the topics 

 under consideration ; in the second place, it is 

 of ereat importance that the author, in the ex- 

 tended treatment of the subject, should not lose 

 sight of its connection with the main stream of 

 advance in physics, both experimental and 

 theoretical. 



The two books under review fulfil these con- 

 ditions in an ideal manner, for they are lioth 

 written by men who have made notable contrilju- 

 tions to our knowledge of the subjects under con- 

 sideration, and have that requisite knowledge of 

 modern physical views to treat the subjects in 

 the right perspective. 



(i) The work of Prof. Bragg deals mainly 

 wifli the phenomena accompanying the passage 

 NO. 2261, VOL. 90] 



of a, /8 and y rays and X-rays through matter. 

 An interesting account is given of the theoretical 

 reasoning that led the author to predict the nature 

 of the absorption of the a rays by matter, and 

 of the admirable experiments made by him which 

 led to such an important extension of our know- 

 ledge of this subject. The results of experiments 

 on the passage of X-rays through matter are 

 interpreted on his well-known corpuscular theory 

 of the X-ray. The essential point of this theory 

 is that the energy of the X-ray is corpuscular in 

 the sense that it is concentrated and does not 

 spread from the source like that to be expected 

 in an ordinary pulse or wave. In addition, it is 

 supposed that the j8 ray and the X-ray are 

 mutually convertible forms of energy. When a 

 & ray disappears as a result of a close encounter 

 with an atom, an X-ray of equal energy takes 

 its place and tends to be propagated in the 

 original direction of the ;3 ray. This theory has 

 the advantage of giving a concrete and easily 

 grasped idea of the processes occurring in the 

 passage of X-rays through matter, and has served 

 a very useful purpose in directing numerous 

 investig'ations which have thrown much lig'ht on 

 the subject. 



It is remarkable that although more than seven- 

 teen years have elapsed since the discovery of 

 X-rays, there is still a great difference of opinion 

 as to their nature and the mechanism of their 

 absorption by matter. The recent striking experi- 

 ments of Laue and his colleagues, supplemented 

 by the explanation of Mr. W. L. Bragg, seem 

 to show conclusively that a fraction of the X-rays 

 suffer regular reflection at the crystal planes of 

 mica and of other crystals. These results seem 

 only compatible with the view that the X-rays 

 are some type of wave motion. On the other 

 hand, the liberation of a high-speed electron from 

 an atom traversed by the X-ray cannot be ex- 

 plained with an)' credibility unless it be supposed 

 that the energy of the X-ray is concentrated over 

 a minute volume, and can be given up in an 

 encounter with a single atom. These apparently 

 conflicting but fundamental properties of the 

 X-ray must be reconciled in any satisfactory 

 theory of the X-rays. 



The book is very pleasantly and clearly written 

 and contains a concise account of most of the j 

 important experiments on the subject under con- 

 sideration. While there will, no doubt, be con- 

 siderable difference of opinion as to the merits i, 

 of the theories proposed by the writer, the book 

 can be strongly recommended not only to the 

 physicist, but to all those who are interested in 

 the fascinating field of inquiry which has been , 



