232 Notices respecting New Boohs. 



almost universal adoption of the latter scale in recent years seems 

 to call for a short note to this passage. 



The chapters on waves and radiation form a good connecting 

 link between the molar and molecular phenomena. An error of 

 the previous editions is repeated on p. 229 of the present one, 

 where the quantity Q of the mathematical expression is stated to 

 be the velocity of propagation of the wave, this latter being really 

 equal to Qv. 



In molecular physics many important advances have been made, 

 and Lord Eayleigh has added some valuable notes on the theory 

 of imperfect gases and of liquids, and on capillarity. Indeed, he 

 may be said to have written a brief history of Laplace's capillary 

 coefficient K, which represents the internal pressure in a gas or 

 liquid resulting from the mutual attractions of its molecules, and 

 which accounts for the observed cohesion of liquids. In evapora- 

 ting a liquid this cohesion is overcome ; and Dupre has shown that 

 K is equal to the energy required to evaporate a cubic centimetre 

 of the liquid (that is, the work-equivalent of the latent heat of 

 unit volume). Tan der Waals and Clausius assumed the existence 

 of this pressure in order to account for the known deviations of 

 gases from the laws of perfect gases, and they have succeeded in 

 explaining these deviations, at any rate for gases and liquids near 

 the critical point. The subject is exciting great interest at the 

 present time, and many readers will be acquainted with the dis- 

 cussion in recent numbers of ' Xature ' between Lord Eayleigh and 

 Prof. Tait, concerning the proper application of Clausius's " equa- 

 tion of the virial " to the case of a gas or liquid with mutually 

 attractive molecules of finite size. Lord Eayleigh, in his note on 

 the virial and its applications, in the volume before us, steers quite 

 clear of any controversial matter ; indeed, we notice that he is as 

 cautious as Maxwell was in treating as a mere assumption the 

 view that the temperature of a liquid is measured by the mean 

 kinetic energy of its molecules. In a recent paper Tait gives 

 strong reasons to the contrary, derived from considering the 

 equality of temperature in the various horizontal layers of a tall 

 column of liquid under gravity. We venture to hope that when 

 the eleventh edition of Maxwell's treatise appears, Lord Eayleigh 

 will be in a position to give us more definite information on some 

 of these knotty points, and that the advance in our knowledge 

 will be such that he can add another ten pages of equally valuable 

 matter. James L. Howard. 



