( 477 ) 



solution ill ineliiyl uk'ohol and precipitation witli ether, when it gave 

 good analvtifal results. ()n being cooled, it became solid, and on 

 inoculation with a trace of the solid material, it yielded beautiful 

 crystals, melting at 41°. The iS-aminojjrojHouic amide 'NR^.Cti.^.Cll.^. 

 CO.NHj is very hygroscopic, very soluble in alcohol, with difficulty 

 in ether, has a strongly alkaline reaction, absorbs carbon dioxide 

 from the atmosphere and yields a well-crystallised hydrochloride. 



Both aminopropionic amides are, therefore, now known and not 

 identical with the substance obtained by Baumstakk from urine. 



Physics. — ''Remarks concerning the dynamics of the electron'' 

 l\y Prof. J. D. VAN der Waals Jr. (Communicated by Prof. 



J. D. VAN UER Waals). 



The theory of electrons is usually deduced from the following 

 equations : 



div i> = () (/) 



1 . 



roU) =: — (b + (> ü) (//) 



c 



1 . 

 rot\i— \) {Ill) 



c 



div{)~ ^ {IV) 



f=ö+-[v()l {V) 



The units and notations used are those of Lorentz's article on the 

 "Elektronentheorie" in the "Encyclopadie der Mathematischen Wis- 

 senschaften" V 14. 



The equations (I) . . . (IV) determine the held, tiie motions of the 

 electrons being given. Equation (V) will be independent of the for- 

 mer four, and determijie the motion of the electron in the electro- 

 magnetic field, ()f representing the force exercised by the field on the 

 electric charge. The application of equation (V) is however attended 

 by peculiar difticulties. If in mechanics a body is given its mass is also 

 supposed to he known ; then if we know the force, the acceleration, 

 and the law according to which it will move may be calculated. 

 Now if in forming f we take into account all the forces also those 

 excited by the electron itself, then the case that we ascribe a "real" 

 or "material" mass to the electron offers no fundamental difficulty. 

 For the case, however, in which the electron has no real mass the 

 equation (V) assumes the form : 



force = 



33- 



