252 Dr. C V. Burton on the Possible Dependence of 



side o£ the oxide layer, and with a strong electric field tending 

 to drag them away from the cathode. 



(3) The chemical combination o£ calcium and oxygen does 

 not by itself give rise to any detectable electron emission. 



From (2) and (3) it appears that neither the theory of 

 Fredenhagen nor the modification of that theory proposed, 

 by Gehrts can be accepted as an explanation of the activity, 

 of the Wehnelt cathode. 



The Cavendish Laboratory, 

 Cambridge. 



XXIX. The Possible Dependence of Gravitational Attraction 

 on Chemical Composition, and the Fluctuations of the Moons 

 Longitude which might result therefrom. By C. V. Burton, 

 JD.Sc* 



1. HMO what degree of accuracy is the Newtonian coefficient 

 A. of gravitation a universal constant of matter inde- 

 pendent of chemical and physical properties ? To decide 

 this question, Newton himself made experiments on a number 

 of substances; and later Besself, as the result of a lengthy 

 investigation, found that, w r ithin the limits of experimental, 

 error (one part in 60,000), the weights of bodies were pro- 

 portional to their masses ; the substances examined including 

 brass, iron, zinc, lead, silver, gold, meteoric iron, meteoric 

 stone, marble, clay, and quartz. Bessel, like Newton, used a 

 pendulum method. 



2. In modern determinations of the acceleration due io 

 gravity, if observations were repeated at a given station with 

 pendulums of different materials, far greater accuracy could 

 be attained ; and additional refinements could no doubt be 

 introduced where the comparative behaviour of different 

 pendulums was the sole subject of research. But experi- 

 menters are not readily attracted to laborious tasks from 

 which only null results are expected. 



3. The planets of the solar system, differing considerably 

 in mean densit} r amongst themselves, may be supposed to 

 differ correspondingly in composition, so that the exactitude 

 with which their orbits conform to Kepler's third law is 

 reasonably regarded as evidence of the close uniformity of 



* Communicated by the Author. 



t " Versuche liber die Kraft mit welcher die Erde Korper von 

 verschiederier Beschaffenheit anzieht." Berlin A bhandl. 1830, pp. 41-102 ; 

 Pogg. Ann. xxv. 1832, pp. 401-417, reprinted in Astron. Nachr. x.. 

 1833, col. 97-108. 



