KONINKLIJKE AKADEMIE VAN WETENSCHAPPEN 

 TE AMSTERDAM. 



PROCEEDINGS 



VOLUME XVII 

 N°. 4-5. 



President: Prof. H. A. LoRENTZ. 

 Secretary: Prof. P. Zeeman. 



(Translated from: Verslag van de gewone vergadering der Wis- en 

 Natuurkundige Afdeeling, Dl. XXIII and XXIV). 



CONTENTS. 



H. A. BROUWER: "Pneumatolytic hornfcis from the hill countries of Siak (Sumatra)". (Communicated 



by Prof. G. A. F. MOLENQRAAFFi, p. 584. 

 H. K. DE HAAS: "A Confirmation of the Principle of Relativity." (Communicated by Prof. H. A. 



LORENTZi, p. 591. 

 F. M. Jaeger and Jul. KahN: "Investigations on the Temperature-Coefficients of the Free Mole- 

 cular Surface-Energy of Liquids from —80° to 1650° C. Xlll. The Surface-Energy of position- 



isomeric Benzene-Derivatives", p. 595. 

 ". M. Jaeger and Jul. KahN: "Ibid. XIV. Measurements of a Series of Aromatic and Heterocyclic 



Substances", p. 617. 

 W. H. Keesom: "The second virial coefficient for rigid spherical molecules, whose mutual attraction 



is equivalent to that of a quadruplet placed at their centre." (Communicated by Prof. H. 



KAMERLINGH ONNES), p. 636. 

 H. A. VERMEULEN: "The vagus-area in Camelopardalus Giraffe". Communicated by Prof. C. 



WiNKLERi, p .647. 

 1. K. A. WERTHEIM SalomonsoN: "A difference between the action of light and of X-rays on the 



photographic plate", p. 671. 

 E. F. VAN DE Sande BakhuvZEN and C. DE JONG: "On the influence exercised by the systematic 



connection between the parallax of the stars and their apparent distance from the galactic 



plane upon the determination nf the precessinnal constant and of the systematic proper 



motions of the stars", p. 683. 

 A. EINSTEIN and W. J. DE Haas: "Experimental proof of the existence of Ampere's molecular 



currents." (Communicated by Prof. H. A. LORENTZI, p. 696. 

 P. ZEEMAN: "On a possible influence of the FRESNEL-coefficient on solar phenomena', p. 711. 

 L. BOLK: "On the Relation between the Dentition of Marsupials and that of Reptiles and Mono- 



delphians", p. 715. 

 W. E. RiNQER: "Further researches on pure pepsin". (Communicated by Prof. C. A. Pekelharing . 



p. 738. 

 A. W. K. DE JONO: "The action of sun-light on the cinnamic acids', p. 751. 

 O. VAN ROMBURGH: "Nitro-derivatives of alkylbenzidines '. (Communicated by Prof. P. VAN ROM- 



BURQH', p. 757. 

 j. DROSTE: "On the field of two spherical fixed centres in ElNSTElNs theory of gravitation". 



iCommunicated by Prof. H. A. LoRENTZi, p. 760. 

 J. G. RUTGERS: "On a linear integral equation of VOLTERRA of the first kind, whose kernel contains 



a function of Bessel'. iCommunicated by Prof. W. KAPTEYN). p. 769. 

 H. A. VERMEULEN: "On the conus mcdullaris of the domestic animals". iCommunicated by Prof. 



C. WiNKLERi, p. 780. 



38 



Procoeding.s Royal Acad. Ainsiti'rdnm. Viil. XVIII. 



