66 Royal Society : — Mr. W. Croakes on the 



weighing substances which were of a higher temperature than the 

 surrounding air and the weights. There appeared to be a diminu- 

 tion of the force of gravitation ; and experiments were instituted 

 to render the action more sensible, and to eliminate sources of error. 



In an historical resume of the state of our knowledge on the sub- 

 ject ot attraction or repulsion by heat, it is shown that in 1792 

 the Bev. A. Bennet recorded the fact that a light substance de- 

 licately suspended in air was attracted by warm bodies : this he 

 ascribed to air-currents. When light was focused, by means of a 

 lens, on one end of a delicately suspended arm, either in air or in 

 an exhausted receiver, no motion could be perceived distinguish- 

 able from the effects of heat. 



Laplace spoke of the repulsive force of heat. Libri attributed 

 the movement of a drop of liquid along a wire heated at one end, 

 to the repulsive force of heat ; but Baden Powell did not succeed 

 in obtaining evidence of repulsion by heat from this experiment. 



Fresnel described an experiment by which concentrated solar 

 light and heat caused repulsion between one delicately suspended 

 and one fixed disk. The experiment was tried in air of different 

 densities ; but contradictory results were obtained under apparently 

 similar circumstances at different times, and the experiments were 

 not proceeded with. 



Saigey described experiments which appeared to prove that a 

 marked attraction existed between bodies of different temperatures. 



Forbes, in a discussion and repetition of Trevelyan's experi- 

 ment, came to the conclusion that there was a repulsive action ex- 

 ercised in the transmission of heat from one body into another 

 which had a less power of conducting it. 



Baden Powell, repeating Presnel's experiment, explained the 

 results otherwise than as due to repulsion by heat. By observing 

 the descent of the tints of Newton's Bangs between glass plates when 

 heat was applied, Baden Powell showed that the interval between 

 the plates increased, and attributed this to a repulsive action of heat. 



Faye introduced the hypothesis of a repulsive force of heat to 

 account for certain astronomical phenomena. He described an 

 experiment to show that heat produced repulsion in the luminous 

 arc given by an induction-coil in rarefied air. 



The author describes numerous forms of apparatus successively 

 more and more delicate, which enabled him to detect and then to 

 render very sensible an action exerted by heat on gravitating bodies, 

 which is not due to air-currents or to any other known form of 

 force. 



The following experiment with a balance made of a straw beam 

 with pith-ball masses at the ends enclosed in a glass tube and con- 

 nected with a Sprengel pump, may be quoted from the paper : — 



" The whole being fitted up as here shown, and the apparatus 

 being full of air to begin with, I passed a spirit-flame across the 

 lower part of the tube at b, observing the movement by a low-power 

 micrometer ; the pith ball ( a, b) descended slightly, and then im- 

 mediately rose to considerably above its original position. It 



