350 The Mechanical Action of Light. [July, 
I have long been experimenting in the endeavour to trace 
some connection between the movements of attraction and 
repulsion above alluded to and the action of gravitation in 
Cavendish’s celebrated experiment. The investigation is not 
sufficiently advanced to justify further details, but I will give 
here an outline of one of the results. 
I find that a heavy metallic mass, when brought near a 
delicately suspended light ball, attracts or repels it under the 
following circumstances :-— 
I. When the ball is in air of ordinary density. 
a. If the mass is colder than the ball, it repels the 
ball. 
b. If the mass is hotter than the ball, it attracts the 
ball. 
Il. When the ball 1s in a vacuum. 
a. If the mass is colder than the ball, it attracts the 
ball. 
b. If the mass is hotter than the ball, it repels the 
ball. 
The density of the medium surrounding the ball, the 
material of which the ball is made, and a very slight differ- 
ence between the temperatures of the mass and the ball exert 
so strong an influence over the attractive and repulsive force, 
and it has been so difficult for me to eliminate all interfering 
actions of temperature, electricity, &c., that I have not yet 
been able to get distinct evidence of an independent force 
(not being of the nature of heat or light) urging the ball and 
the mass together. 
Experiment has, however, shown me that, whilst the 
action is in one direction in dense air, and in the opposite 
direction in a vacuum, there is (as I have already pointed 
out in the experiments described in the commencement 
of this paper) an intermediate pressure at which differ- 
ences of temperature appear to exert little or no inter- 
fering action. By experimenting at this critical pressure, 
and at the same time taking all the precautions which 
experience shows are necessary, it would seem that such an 
action as was obtained by Cavendish, Reich, and Baily 
should be rendered evident. 
It is not unlikely that in the experiments here recorded 
may be found the key of some as yet unsolved problems in 
celestial mechanics. In the sun’s radiation passing through 
