591 



cially of the alkalies), vvliich occurred in sufficient quantity to make tlie 

 magma cryslaiJise as a granite, but not in sufficient quantity to 

 cause felspatliisalion in the adjoining rocks. To match this supposi- 

 tion, the large extension of the granitic area of Rokan and the 

 occasional occurrence of small outcrops of granite in the hill countries of 

 Siak point to the fact that in the first mentioned area the granite 

 and the contact-rocks have been uncovei'ed to a lower level by erosion. 



Physics. — "A Coafirnintton of the Principle of Relativif;/" ■ By 

 Dr. H. K. DE Haas. (Commnnicaled by Prof. H. A.Lorentz). 



(Ilommunicaled in tlie meeting of June 26, 1915). 



The following considerations founded on a negative result of an 

 experimental research') concerning the question: "does gravitation 

 require time for its extension in space?" corroborate one of the 

 principal theses of the principle of relativity with a greater degree 

 of accuracy, than is possible for light. Any effect resulting at any 

 moment from the relative motion of matter and ether, diame- 

 trically opposed to the motion of 30 km. + or — the component 

 of the motion toward the apex in the direction of these 80 km. 

 per second twelve hours later, can be excluded as regards gravi- 



/ 1 \'% 

 tation, with an accuracy of ,-— - 



At the extremities of a torsion-balance two balls of e(pial weight 

 were hung, one of platinum ( s[). gr. : 21-5), the other of paraflin 

 (sp. gr. : 0-87). 



The constants of the apparatus were: 



Weight: Grams: Moment of inertia (^cnr. gr.). 



Platinum ball 11 -6628 2189 



Paraffin ball 11-6612 2199 



Beam 2 1670 145 



2 hooks (at ends of beam) 2 X 0-0364 14 



Suspension-hook -\- mirror ± 3 



Q = 4550 

 The distance from the hooks at the extremities to the (middle) 

 suspension hook: 13'70 cm. ±001 



The half period : {f^ = 340" ± 1. 



1) For details see: Reports of the lectures delivered by members of the i>atott/"sc/i 

 Genootschap at Rotterdam. Vol. 1, 1915. 



