KONINKLIJKE AKADEMIE VAN AVETENSOHArrEN 

 TE AMSTERDAM. 



PROCEEDINGS OF THE MEETING 

 of Saturday September 30, 1911. 



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

 Afdceling van Zaterdag 30 September 1911, Dl. XX). 



C03srTE3vrTS. 



,T. 1) VAN UKU Waals Jr.: "Energy and mass". (Connnunicated by l'rof .t. D. van dkii Waai.s), 



p. 239. 

 Jan de Vkiks: "A bilinear congruence ui' ([uartii- twisted curves of tlie first species", ]). 255. 

 Jax de Vries: "A (juartic surface with twelve straiglit lines'', p. 259. 



A. S.MiTS : "The application of the new tlieory of allotropy to the system sulphur". (Commu- 

 nicated by Prof. A. F. IIoixemvn), p. 263. 

 Pii. KOHNSTAMM and J. CiiR. Eeeders: "On the phenomena of condensation for mixtures of 



carbonic acid antl nitrobenzene in connection with double retrograde condens.ition". 



(Communicated by Prof. J. D. van deu TVaai.s), p. 270. 

 J. .T. VAN Laar: "The variability of the (quantity /; in van der Waals' etjuation of state, also 



in connection with the critical quantities". I. (Communicated by Prof. H. A. Lorkntz), 



p. 278. 

 T. VAN DER ]<1NDEN: "On the benzeuchexachloridcs and their splitting up into trichloreben- 



zenes". (Communicated by Prof. A. P. Holle.man), p. 298. 



Physics. — ''Eiiergij ami mass'. By .). D. van dek Waals Jr. 

 (Communicated by Prof. J. D. van der Waals). 



§ i. Inirodtiction. In chissical mechanics the mass of the bodies 

 was considered to be constant and tlie force was defined as the time 

 derivative of tiie momentum; in consequence of this the law of 

 conservation of momentum, the hiw : the action is e(|ual to the 

 reaction, and the law of the uniform motion of the centre of inertia 

 of an isolated system were considered to be tiiree different ways to 

 state the same law of nature. At present many pliysicists consider 

 this not to be the case. They assume that the hiw of conservation 

 of momentum iiolds <2;ood in nature. Foi' this assuuiptiou, liowever, 

 it is necessary to ,<>encraHze tlie notion of moment urn, so thnt we 

 ascribe also a (|uaiilify of monieiiliiui lo the electromagnetic Held. 

 The law action = reacliou however, most physicists consider not 



17 



Proceedings Royal Acad. Amsterdam. Vol. XIV. 



