134 



G. J. EliaS: "On a General Electromagnetic Thesis and its Application to the Magnetic State of a 

 Twisted Iron Bar". (Communicated by Prof. H. A. LORENTZ), p. 327. 



A. Smits : "Molecular-Allotropy and Phase-Allotropy in Organic Chemistry". (Communicated by Prof. 

 J. D. VAN DER WAALS), p. 346. 



A. Smits: "The Apparent Contradiction between Theory and Practice in the Crystallisation of AUotropic 

 Substances from Different Solvents". (Communicated by Prof. J. D. VAN DER WAALS), p. 363. 



Devendra Nath Bhattacharyya and Nilratan Dhar : "Supersaturation and release of super- 

 saturation". (Communicated by Prof. ERNST COHEN), p. 369. 



Devendra Nath Bhattacharyya and Nilratan Dhar: "Temperature-coefficient of conductivity 

 in alcoholic solutions, and extension of Kohlrausch's hypothesis to alcoholic solutions". (Com- 

 municated by Prof. Ernst Cohen), p. 373. 



Devendra Nath Bhattacharyya and Nilratan Dhar: "Velocity of ionsatO°C." (Communicated 

 by Prof. Ernst Cohen), p. 375. 



Nilratan Dhar: "Properties of elements and the periodic system". (Communicated by Prof. ERNST 

 Cohen), p. 384. 



P. Zeeman : "Fresnel's coefficient for light of different colours". (Second part), p. 398. (With one 

 plate). 



H. Kamerlinqh Onnes, C. Dorsman and G. Holst : "Isothermals of diatomic substances and their 

 binary mixtures. XV. Vapour pressures of oxygen and critical point of oxygen and nitrogen". 

 (Errata to Comni. N°. 145ft from the Physical Laboratory at Leiden, Jan. 1914), p. 409. 



Physics. — "The in/dtJi of spectral Unes." By Prof. H. A. Lorentz. 



(Communicated in tlie meeting of June 27, 1914). 



§ 1. In order to account for the absorption of light we maj' 

 suppose the molecules to contain electrons which are set vibrating 

 by the incident rays and experience a resistance to their motion. 



If we suppose that an electron is drawn towards its position of 

 equilibrium by a quasi-elastic force and that the resistance is pro- 

 portional to the velocity, the vibrations are determined by the 

 equation 



i>'r = —/r — gr + eE (1) 



where the vector r means the displacement from the position of 

 equilibrium and E the electric force in the incident light. The mass 

 and the charge are represented by m and e, whereas /' and ƒ/ 

 are the constants for the quasi-elastic force and the resistance. 



The theory takes its simplest form for a gaseous body of not too 

 great a densitj'; to this case I shall here confine myself. If there 

 are several groups of electrons, those which belong to the same 

 group being equal and equally displaced, we may write for the 

 electric moment per unit of volume 



P = V A^ e r , (2) 



where the sign 2! refers to the different groups and N means the 

 number of electrons per unit of volume for each group. The di- 

 electric displacement is given by 



D = E+ P 

 and in addition to these formulae we have the general equations 



