RECENT ADVANCES IN SCIENCE 21 



that it allows one to formulate a Lagrangian function leading 

 by an application of the Lagrange equations to Maxwell's 

 equations for the field. The paper is subjected to some 

 criticism by Dr. G. H. Livens in the March number of the 

 Phil. Mag. 



In the February number of the Phil. Mag. Lord Rayleigh 

 discusses the methods employed for detecting small optical 

 retardations, and the test introduced by Foucault and modified 

 by Cheshire for discovering lack of perfection in optically 

 figured surfaces. 



In Nature of March 1, Dr. Todd proposes an interesting 

 thermodynamical explanation of the temperature coefficient 

 of the gravitational constant, evidence for which Dr. Shaw 

 has recently claimed to discover. [See Science Progress,, 

 Jan. 191 7 ; Rec. Adv.] The fundamental assumption is that 

 a movement of an attracted mass to or from an attracting- 

 body involves a change of temperature. A cycle of the Carnot 

 type can be constructed, and an expression for the attraction 

 between two masses developed which contains a term depending- 

 on temperature in addition to the usual Newtonian formula. 

 Some interesting discussion is printed in the following numbers 

 of Nature. 



In the January number of the Physical Review appears a 

 theoretical paper by A. H. Compton on the intensity of X-Ray 

 reflection which, among other results, supports the theory 

 of the atom propounded by Bohr. 



In the same number, C. D. Child gives an account of some 

 work, supporting the view that light is produced by the v re- 

 combination of ions, or, in other words, that the vibrations 

 of the atoms producing light are not started when the atoms 

 are broken into ions, but when the ions recombine into atoms. 



PHYSICAL CHEMISTRY. By Prof. W. C. McC. Lewis, M.A., D.Sc. 

 University, Liverpool. 



Osmotic Pressure {continued). — We now come to the problem 

 of how to account for the phenomenon of osmotic pressure at 

 all. For the sake of simplicity we shall restrict ourselves to 

 the ideal solution ; the ideal solution being one in which the 

 volume of the mixture is exactly equal to the sum of the volumes 

 of the two constituents separately. In such a case the funda- 



