ESSAY-REVIEWS 655 



the tangential traction and of the passage of heat. The question whether 

 this is so or not is left open, but it is declared not to be probable. 



The papers on Sound teem with suggestive experiments and observations, 

 whether they are dealing with the character of the S sound, aeolian tones, 

 fog-signals, the perception of sound, the resonant reflexion of sound from a 

 perforated wall, or the propagation of sound in water. Take the paper on 

 the last subject for example (No. 414), which summarises in a succinct way 

 the chief eSects of the free surface, distinguishing between this case and that 

 of reflexion from a wall in air. 



In connection with Thermodynamics there occur only four papers, but 

 they are of great interest. In a letter to Nernst (No. 356) he returns to the 

 question of the failure of the law of equipartition of energy. " Perhaps this 

 failure might be invoked in support of the views of Planck and his school 

 that the laws of dynamics (as hitherto understood) cannot be applied to the 

 smallest parts of bodies. But I must confess that I do not like this solution 

 of the puzzle. Of course I have nothing to say against following out the 

 consequences of the [quantum] theory of energy. . . . But I have a dif&culty 

 in accepting it as a picture of what actually takes place." This was written 

 in 1911. 



In a paper (No. 379) on the pressure of radiation and Carnot's principle 

 he shows in detail, by means of a Carnot's cycle in which radiation is the 

 substance operated with, that the second law must be violated if no radiation- 

 pressure exists. Bartoli's proof " employs irreversible operations," and 

 " does not lend itself to further developments." 



Article 418 contains a timely exposition of the La Chatelier-Braun principle 

 so much employed by physical chemists. " As usually formulated, the prin- 

 ciple is entirely ambiguous." " Nothing definite can be stated without a 

 discrimination among the parameters by which the condition of a system 

 may be defined," " Returning to the compressed gas, we now recognise that 

 it is the pressure, 8p, which is the force and —8v the effect." " But we may 

 still feel a doubt as to which is the strained condition, the isothermal or the 

 adiabatic, and without a decision on this point no statement can be made." 

 It is, however, evident that if the general theorem is applicable at all, the 

 adiabatic condition must be regarded as the constrained one, since the response 

 is to be diminished by a constraint. 



The volume contains a large number of papers in the region of Optics. 

 Two are on the propagation of waves in stratified media. In the second 

 (No. 422) the case is taken of a succession of equally spaced parallel plates 

 of equal thickness — a succession of Haidinger plates, in fact. This differs 

 from the problem of a pile of plates discussed by Stokes, inasmuch as attention 

 is paid to phase as well as amplitude. The reflexion coefficient for a single 

 plate being r (where r is a complex quantity such that multiplied by its 

 conjugate the reflected intensity is obtained), and the transmission coefficient 

 is t, then it is shown that for any number, n, of plates the corresponding 

 quantities are — 



for reflexion 4>.= ^^,y2 ^L^ '\K~ f] 

 ^ r {{q + I) P" - ip + i)q''} 



where p-{-q='r^ — t^— i and pq — f, 

 and for transmission 



The problem is worked out on account of its application to the light 

 reflected from certain chlorate-of-potash crystals which are multiple-twinned. 

 For favoured wave-lengths it is shown that the reflected light is 73 per 



