1900.] on Recent Studies in Gravitation. 287 



between two bodies, tbe lines of force will move into it from eacb 

 side, and the gravitative pull on a body, near the interposed medium 

 on the side away from the attracting body, will be increased. 



The apparatus they used was a modified kind of Boys' apparatus 

 (Fig. 8). Two small gold masses in the form of short vertical wires, 

 each -i gm. in weight, were arranged at different levels at the ends 

 virtually of a torsion rod 8 mm. loug. The attracting masses M l M 2 

 were lead, each about 1 kgm. These were first in the positions 

 shown by black lines in the figure, and were then moved into the 

 positions shown by dotted lines. The attraction was measured first 

 when merely the air and the case of the instrument intervened, and 

 then when various slabs, each 3 cm. thick, 10 cm. wide and 29 cm. 

 high, were interposed. With screens of lead, zinc, mercury, water, 

 alcohol or glycerine, the change in attraction was at the most about 

 1 in 500, and this did not exceed the errors of experiment. That is, 

 they found no evidence of a change in pull with change of medium. 



Fig. 7. — Effect of interposition of more permeable 

 medium in radiating field of force. 



If such change exists, it is not of the 'order of the change of electric 

 pull with change of medium, but something far smaller. Perhaps, 

 it still remains just possible, that there are variations of gravitational 

 permeability comparable with the variations of magnetic permea- 

 bility in media such as water and alcohol. 



Yet another kind of effect might be suspected. In most crystal- 

 line substances the physical properties are different along different 

 directions in a crystal. They expand differently, they conduct heat 

 differently, and they transmit light at different speeds in different 

 directions. We might, then, imagine that the lines of gravitative 

 force spread out from, say a crystal sphere unequally in different 

 directions. Some years ago, Dr. Mackenzie* made an experiment in 

 America in which he sought for direct evidence of such unequal 

 distribution of the lines of force. He used a form of apparatus like 

 that of Professor Boys (Fig. 2), the attracting masses being calc 

 spar spheres about 2 inches in diameter. The attracted masses in 

 one experiment were small lead spheres about ^ gm. each, and he 



* Physical Keview, ii. 1895, p. 321. 



