272 KINETIC THEORIES OF GRAVITATION. 



sess, and which they are all supposed to radiate in exchange, will cause 

 all bodies to be urged toward one another."'* 



This hypothesis would make gravitation, or molecular attraction, a 

 function of temperature, contrary to all observation ; and is indeed en- 

 tirely incompatible with the fourth, fifth, and sixth conditions of gravi- 

 tative action. In short these experiments, striking and instructive as 

 they unquestionably are, will be found on careful scrutiny to really sim- 

 ulate the effects of gravity in no one particular. 



Crookes. 1873. 



From some very ingenious and interesting experiments made by 

 William Crookes, in 1873, with light disks attached to a delicate torsion- 

 balance and hermetically sealed within a nearly perfect vacuum, in 

 which experiments the radiation of heat was observed to exert a repul- 

 sive effect upon the blackened side of the disks, he inferred that a clue 

 was thereby probably furnished to the mystery of gravitation. He thus 

 concludes his first memoir : "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 the quasi- 

 vacuum of space we have the radial repulsive force, possessing success- 

 ive propagation, required to account for the changes of form in the 

 lighter matter of comets and nebulse ; . . . . but until we meas- 

 ure the force more exactly, we shall be unable to say how much influ- 

 ence it may have in keeping the heavenly bodies at their respective dis- 

 tances. So far as repulsion is concerned, we may argue from small 



things to great, from pieces of pith up to heavenly bodies 



Although the force of which I have spoken is clearly not gravity solely, 

 as we know it, it is attraction developed from chemical activity, and 

 connecting that greatest and most mysterious of all natural forces, 

 action at a distance, with the more intelligible acts of matter. In the 

 radiant molecular energy of solar masses may at last be found that 

 ' agent acting constantly according to certain laws,'t which Newton held 

 to be the cause of gravity." | 



Similar expositions are announced in Mr. Crookes' Journal for 1875, § 

 and various modifications of the apparatus employed in the experiments 

 are detailed. These little instruments, inclosed within an exhausted 

 bulbous tube of glass, and with the disk-arms mounted on a pivot to 

 permit continuous rotation, have since become quite familiar under the 

 name of " radiometers," and have received a careful investigation from 

 a number of observers. It is now known that they do not act by any im- 

 pulsion of radiation, but solely by the differences of heat-absorption by 



* L. E. D. Phil. Mag., November, 1870, vol. xl, p. 354. 

 t Referring, of course, to the " Third Bentley Letter." 

 t Philoaoph. Trans. Roy. Soc, 1874, vol. clxiv, p. 527. 

 $ Quarterly Journal of Science, July, 1875, vol. v, p. 351. 



