lló 



The fundamental curve of A is apparently identical with the 

 curve f J («5» 5) belonging to the point T; we shall indicate it by a'. 

 As c.^ has two pairs in common with t' (§ H) .1 is a node of «". 

 That ('.' passes tlirough the points 6'/, and has triple points in Bk 

 ensues from the consideration of the lines TM/, and of the tangents 

 out of T to the (.lOs belongino- to Bj,. 



It appears analogously that the fundamental curve of C^ has 

 triple points in Bj; and a node in d ; it passes through A and the 

 remaining points Ci, and is of order sir. This curve is at the same 

 time the f^* belonging to M^. 



We can now prove once more that the birational correspondence 

 is of onh'i' 35. To the intersection .Y of two lines /, corresponds 

 the poijit J", which the two curves /, apart from the fundamental 

 points, have in common. As appears from what was mentioned 

 above / passes '18 limes through Bk and 6 times through A and 

 C\; fi-om 1 + 3 X IS'' + 7 X 6' = 1225 = 35^ it appears now that 

 yl is a curve of order 35. 



Physics. — -'On the marmer in lohich the susceptihiUtii of para- 

 ma</nefic substances depends on the density." By Dr. W. H. 

 Keesom Supplement N". 36r to the Comnuinications from the 

 Physical Laboiatory at Leiden. Comuuiuicated by Prof. H. 

 Kamerlingh Onnes. 



(Gommunicaled in the meeling of April 24, 1914). 



§ 1. Introduction. In Suppl. N". 32a (Oct. '13) an expression was 

 dexeloped for the molecular rotatory energy in a system of freely 

 rotating molecules as a function of the temperature. This expression 

 was introduced into tlie theories of LatsGF.vin and Weiss, on the sup- 

 position that, when the equipartition laws are deviated from, the 

 statistics of the molecules under the action of an exterior directing 

 field, in this case a magnetic tield, is determined by the value z^i- of 

 the rotatory energy in the same way as for equipartition it is by 

 kT. It then appeared that ditferent experimental results can be re- 

 presented very satisfactorily in that way 



\r 1^ 



1) The expressions developed in the above-mentioned paper appear to be also 

 suitable to give a quantitative representation (as far as observations are available) 

 of the decrease of the temperature of the GuRiE-point by the addition of a dia- 

 magnetic metal to a ferromagnetic one, with which it forms mixed crystals, on 

 the supposition that the diamagnelic metal exerts no other influence than that the 

 mutual action of the ferromagnetic molecules is lessened in consequence of the 



