RECENT ADVANCES IN SCIENCE 383 



same distance and follows rigorously the inverse square law. 

 The result will be the same as if the inertial mass remained 

 constant and the intensity of gravitational attraction were 

 increased slightly during one half a revolution. The result 

 will be a " slewing round " of the orbit independent of the 

 perturbations of other planets. The author subjects the 

 matter to rigorous analysis and after making a reasonable 

 assumption for the value of a constant succeeds in accounting 

 for the discrepancy referred to above. Unfortunately, as Prof. 

 Eddington points out in the two succeeding numbers of the Phil. 

 Mag., while Lodge's method resolves the discrepancy existing 

 between observation and Newtonian Theory in the case of the 

 perihelion of Mercury's orbit, it also introduces discrepancies 

 in the case of Venus and the Earth which do not exist. 

 Einstein's Theory, with all its difficulty and its revolutionary 

 conceptions, is free from this defect. 



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

 University, Liverpool. 



Molecular Heats of Gases. — Bjerrum (Zeitschr. Elektrochem. 

 191 1, 17, 731) appears to have been the first to consider in some 

 detail the question of the molecular heats of gases, by taking 

 into account the motion of the molecule as a whole and the 

 motions of the constituent atoms. It has long been recognised 

 that a material body is capable in general of three types of 

 motion, viz. motion in respect of translation, rotation and 

 vibration. To each type there corresponds a certain number 

 of degrees of freedom. In the case of translation the number 

 of degrees of freedom is known to be three, and, corresponding 

 to this, the molecule of any gas must possess energy to the 

 amount 3/2 RT. This is true whether the molecule be mon- 

 atomic or polyatomic. It follows that the minimum value for 

 the molecular heat of any gas is 3/2 R. or 2*98 calories per 

 degree. The measurements of Pier, in the case of the mon- 

 atomic gas argon, have shown that this substance actually pos- 

 sesses the value 2' 98, and, further, this quantity is independent 

 of the temperature. It follows, therefore, that a monatomic 

 molecule does not possess any rotational energy. It is obvious, 

 of course, that it is incapable of vibration. Since the mon- 

 atomic molecule, i.e. the atom itself, is evidently incapable of 



