Physics. — "SoJiie experiments on gravitation. The ratio 0/ mass 

 to weight for crystals and radioactive substances." By Prof. 

 P. Zeeman. 



(Communicated in the meeting of Sept. 29, 1917). 



i. Our ideas concerning gravitation have been so radically 

 changed by Einstein's theory of gravitation that questions of the ut- 

 most interest in older theories are now simpl\ discarded or at least 

 appear in a changed perspective. We cannot try anymore to form 

 an image of the mechanism of the gi-avitational action between two 

 bodies, and we must return to the older theories in order to justify 

 the suspicion, that the structure of substances might influence their 

 mutual attraction. In most crystalline substances the velocity of 

 propagation of light, the conduction for heat and the dielectric con- 

 stant are different in different directions, and we might then suspect 

 that the lines of gravitativ'e force spread out from a crystal un- 

 equally in different directions. 



A. S. Mackenzie^) in America, and Poynting and Gray^) sought 

 for evidence of a directive gravitational attraction. 



Mackenzie proved with an apparatus like that used by Boys in 

 his beautiful researches on the gravitation constant, that when the 

 axes of calcspar spheres were set in various positions the maximum 

 difference of attraction amounted to less than -g-o "^'^ P^i't of the 

 total attraction. 



Poynting and Gray proved that the attraction between two quartz 

 spheres with parallel axes, differs less than 1 in 16000 from the 

 attraction between these spheres with crossed axes. 



Kreichgauer^) sought for a change of weight of sodium acetate 

 when this substance crystallized from the fluid (supersaturated) 

 state. It appeared that the change amounted to less than A.10~^ of 

 the total weight. 



2. The weight of quartz spheres in different positions. 

 Determinations of the weight of crystals in different orientations 

 have, I believe, never been published. Some years ago I decided upon 



M Physical Review 2. 321. 1895. 



2) Philosophical Transaclions. A. 192. 245. 1899. 



3) Kreichgauer. Verhandl. Berliner Physik. Ges. 10. 18. 1892. 



