REPULSION RESULTING EROM RADIATION. 
523 
practice the beam can be balanced on one point of the suspending-needle, which will 
be nearly vertical. In this position the beam has a horizontal movement ; and by care- 
fully adjusting the level of the tube the delicacy of the beam can be made very much 
superior to what it would be suspended in the ordinary way. By bringing a warm 
body near one end of the balance, it is now driven away to the utmost extent, and a 
piece of ice attracts it with equally marked energy. 
70. On trying this experiment in air of ordinary density, the approach of a hot body 
causes unmistakable attraction, and a cold body repulsion. In a vacuum this mode 
of arranging the apparatus did not at first appear to offer advantages over the plan 
already adopted ; but as from the direction of movement it was not likely that air- 
currents could interfere, or at all events not to any great extent, I have arranged appa- 
ratus for obtaining the movements of repulsion and attraction in a horizontal instead 
of a vertical plane, so as to examine the action in air. 
Instead of supporting the beams on needle-points, so that they could only move 
up and down, I suspend them by the centre to a long fibre of cocoon-silk in such a 
manner that the movements would be in a horizontal plane. With apparatus of this 
kind, using very varied materials for the index, enclosing them in tubes and bulbs of 
different sizes, and experimenting in air and gases of different densities up to Sprengel 
and chemical vacua, I have carried out a large series of experiments, and have obtained 
results which, whilst they entirely corroborate those already described, carry the inves- 
tigation some steps further in other directions. I propose shortly to submit an account 
of this second series of researches to the Society. 
71. I have more recently instituted experiments to ascertain how far the action of 
gravitation in Cavendish’s celebrated experiment is likely to be modified under the 
influence of heat. For many months I have been experimenting with apparatus devised 
for this purpose. The investigation is not sufficiently advanced to justify further details, 
but I may perhaps be permitted to give here an outline of one of the results. 
72. 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. 
II. When the ball is 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. 
73. The density of the medium surrounding the ball, the material of which the ball 
is made, and a very slight difference 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. 
