1873.] On the Action of Heat on Gravitating Masses. 37 



II. " On the Action of Heat on Gravitating Masses." By WILLIAM 

 CROOKES, F.R.S. &c. Received August 12, 1873. 



(Abstract.) 



The experiments recorded in this paper have arisen from observations 

 made when using the vacuum-balance, described by the author in his 

 paper " On the Atomic Weight of Thallium"*, for weighing substances 

 which were of a higher temperature than the surrounding air and the 

 weights. There appeared to be a diminution 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 subject of 

 attraction or repulsion by heat, it is shown that in 1792 the Rev. A. 

 Bennet recorded the fact that a light substance delicately 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 distinguishable 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 re- 

 pulsive force of heat ; but Baden Powell did not succeed in obtaining 

 evidence of repulsion by heat from this experiment. 



Presnel 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 con- 

 tradictory 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 experiment, 

 came to the conclusion that there was a repulsive action exercised 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 Rings 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. 



Eaye 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 induc- 

 tion-coil in rarefied air. 



The author describes numerous forms of apparatus successively more 



* Phil. Trans. 1873, vol. cliiii. p. 277. 



