1873.] Action of Heat on Gravitating Masses. 41 



Speaking of Cavendish's celebrated experiment, the author says that 

 he has experimented for some months on an apparatus of this kind, and 

 gives the following outline of one of the results he has obtained : 



" A heavy metallic mass, when brought near a delicately suspended 

 light ball, attracts or repels it under the following circumstances : 



" I. W7ien the ball is in air of ordinary density. 



a. If the mass is colder than the baD, 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." 



The author continues: " 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 diffi- 

 cult for me to eliminate all interfering actions of temperature, electricity, 

 &c., that I have not yet been able to get distinct evidence of an inde- 

 pendent force (not being of the nature of heat) urging the ball and the 

 mass together. 



" Experiment has, however, showed me that, whilst the action is in one 

 direction in dense air, and in the opposite direction in a vacuum, there is 

 an intermediate pressure at which differences of temperature appear to 

 exert little or no interfering action. By experimenting at this critical 

 pressure, it would seem that such an action as was obtained by Cavendish, 

 Keich, and Baily should be rendered evident." 



After discussing the explanations which may be given of these actions, 

 and showing that they cannot be due to air-currents, the author refers to 

 evidences of this repulsive action of heat, and attractive action of cold, in 

 nature. In that portion of the sun's radiation which is called heat, we 

 have the radial repulsive force, possessing successive propagation, re- 

 quired to explain the phenomena of comets and the shape and changes of 

 the nebulae. To compare small things with great to argue from pieces 

 of straw up to heavenly bodies it is not improbable that the attraction, 

 now shown to exist between a cold and a warm body, will equally prevail 

 when, for the temperature of melting ice is substituted the cold of space, 

 for a pith ball a celestial sphere, and for an artificial vacuum a 

 stellar void. In the radiant molecular energy of cosmical masses may 

 at last be found that " agent acting constantly according to certain laws," 

 which Newton held to be the cause of gravity. 



