278 



SCIENCE. 



[N. S. Vol. XX. No. 504. 



Chwolson's 'Physics,' covering the general 

 subject of mechanics so far as this is required 

 as an introduction to general physics, was re- 

 viewed by the present writer not quite a year 

 ago in Science. The second volume, cover- 

 ing the subjects of sound and radiant energy, 

 abundantly justifies the very favorable opinion 

 at that time expressed. The author keeps 

 before him a well-defined object, the prepara- 

 tion of a text-book for students, but not a 

 handbook for specialists. The aim is that 

 the student may find what he needs, and also 

 that he may need what he finds. The stu- 

 dent's standpoint is continually occupied, 

 though it is assumed that no student would 

 feel prepared to undertake the mastery of 

 such a book in four large volumes without 

 having previously mastered at least the ele- 

 ments of the subject as set forth in ordinary 

 preparatory school work. Nevertheless, in 

 such work it is not uncommon for the ele- 

 mentary student to reach a standpoint that 

 interferes with his subsequent attainment of 

 a comprehensive view. For example, light 

 rays, heat rays and chemical rays are even 

 yet supposed often by beginners to be different 

 in kind, and the gap between these and electric 

 rays is hard to bridge. 



Chwolson undertakes the task of setting the 

 student initially on the right standpoint, and 

 of guarding specially against what his experi- 

 ence has shown him to be the most frequent 

 misconceptions. Mathematics is not avoided, 

 but no unnecessary complexities are ever in- 

 troduced. Indeed, it would be hard to find 

 a book in which good arrangement and ac- 

 curate statement are attained with so little 

 in the way of mathematical difficulties, though 

 no one need expect to attack any of the prob- 

 lems of physical optics without having to 

 grapple with equations that require thought. 



Eather less than the first seventh of the 

 present volume is devoted to the subject of 

 sound. It begins with a chapter on the 

 velocity of propagation of a disturbance in an 

 elastic medium, in which the well-known 

 formula for both longitudinal and transverse 

 vibrations is deduced with unusual simplicity. 

 The last few pages are given to the physical 

 basis of music, which is treated only in out- 



line. In the development of the natural scale 

 the exclusive standard recognized is the 

 French normal A = 435, at present adopted as 

 an international musical standard. Now that 

 Ivonig is dead, the Chladni standard, C = 256, 

 so extensively employed by him for standard 

 forks, may quite probably pass away, as Stutt- 

 gardt pitch, C = 264, has already disappeared, 

 despite its use by Helmholtz in the ' Tonem- 

 pfindungen.' 



In the introduction to the general subject 

 of radiant energy Chwolson goes at once into 

 an exposition of electric radiation, closing the 

 chapter with an enumeration of five divisions 

 based on wave-length, and with a brief refer- 

 ence to ' neue Strahlen,' the rays connected 

 with the names of Eontgen, Becquerel and 

 Curie. These are still so little known that he 

 does not feel warranted in linking the treat- 

 ment of them with that of radiations of meas- 

 urable wave-length. 



The phenomena of luminescence are treated 

 as a special transformation of energy quite 

 apart from those of calorific radiation. Spe- 

 cially good and up to date is the discussion of 

 the rate of emission of a black body as a func- 

 tion of temperature and wave-length. The 

 laws of Stefan and Wien are considered, and 

 the conclusion expressed that Stefan's law is 

 thoroughly reliable, but is applicable only to 

 the total radiation of an absolutely black body. 

 The expressions for radiation of special wave- 

 length obtained by Weber, Paschen, Wien, 

 Thiessen, Eayleigh, Planck and Lummner and 

 Jahnlce are all considered, and as the result of 

 comparison a preference is expressed for that 

 of Planck (1900). The effect of radiation as 

 pressure upon the surface of an absolutely 

 black body, suspected as long ago as 1754 by 

 DuFay, was first proved experimentally in 

 1900 by P. Lebedew, whose apparatus is shown 

 and explained. Due credit is given to Nichols 

 and Hull (1901), who have measured this pres- 

 sure and obtained results that accord well 

 with the requirements of theory. The appli- 

 cation to the phenomena of comets is expressed 

 by the statement that the ,theory is capable 

 of affording a full exijlanation of the observed 

 » forms of comets' tails. E. F. Nichols's great 

 improvements upon the sensitiveness of the 



