302 Dr. G. Bruhat on the Theories of 



Summary . 



§§ 1-2. Statement of the problem. 



§§ 3-5. The experimental arrangements. 



§§ 6-11. The main experimental results which are sum- 

 marized in § 12 (p. 296). 



§§ 13-15. Some subsidiary results. 



§§ 16-18. The interpretation of the results. It is possible 

 to explain the observations by the assumption that the ioni- 

 zation observed at a platinum surface for rays with a speed 

 of less than 40 volts takes place, not in the platinum, but in 

 gas (hydrogen ?) attached to the metal ; but this is not the 

 only possible explanation. Some curious effects observed 

 may possibly be connected with the double layer described 

 by Seeliger. 



It is urged that the results throw light on the experiments 

 which have been described illustrating the abolition of the 

 photoelectric effect when the surface of a metal is " cleaned/' 

 and that they destroy most of the reasons for holding that 

 the prevalent theory of that effect is entirely incorrect. 



Leeds, June 1914. 



XXXVI. On the Theories of the Rotational Optical Activity, 

 To the Editors of the Philosophical Magazine. 

 Gentlemen, — 



IN several papers recently published in the Philosophical 

 Magazine, Mr. G, H. Livens * compares the theories of 

 the optical rotatory power given by I) rude and Lorentz. 

 For my partj, I have attempted to verify experimentally 

 these theories. May I be allowed to set forth for the readers 

 of your Journal the reasons why I prefer Drude's theory ? 



A first test of any theory must be obtained by the experi- 

 mental verification of the formulae it furnishes for the 

 rotatory dispersion. Mr. Livens, in order to explain Prof. 

 Cotton's experiments by the theory of Lorentz, assumes that 

 the absorption band which is taken into account arises from 

 a group of non-rotationally active electrons. This assump- 

 tion cannot be correct : Cotton has indeed shown that there 

 are anomalies of the rotatory dispersion and circular dichroism 

 only when the same molecule is absorbing and optically 

 active J, and I have observed the same phenomena in a pure 



* G. H. Livens, Phil. Mag. xxv. p. 817 ; xxvi. pp. 362, 535 ; xxvii. 

 pp. 468, 994. 



t G. Bruhat, These de Doctorat, Paris, June 1914. 



% A. Cotton, Ann. de Chim. et de Phys. (7) viii. p. 347 (1896). 



