KINETIC THEORIES OF GRAVITATION. 273 



the two sides of the disks and the reactions of the residual air of the 

 exhausted cliamber.* 



In a later essay on " The Mechanical Action of Light," Mr. Crookes 

 estimates that "the pressure of sunshine'' amounts "to 2 cwts. per 

 acre, 57 tons per square mile, or nearly three thousand million tons on 

 the exposed surface of the globe; — sufiflcient to knock the earth out of 

 its orbit if it came upon it suddenly.t It may be said that a force like 

 this must alter our ordinary ideas of gravitation ; but it must be remem- 

 bered that we only know the force of gravity between bodies such as 

 they actually exist, and we do not know what this force would be if the 

 temperatures of the gravitating masses were to undergo A change. If 

 the sun is gradually cooling, possibly its attractive force is increasing, 

 but the rate will be so slow that it will probably not be detected by our 

 present means of research."| 



This possibility is denied by our sixth proposition. The generaliza- 

 tions embraced in our first, second, and third propositions are assumed 

 to be equally true at all possible temperatures ; and the ground for 

 this assumption is, that they have been actually ascertained to be 

 true for all observed variations of temperature. The hiduction can 

 therefore be arraigned only by a conflicting fact of observation or 

 experiment. Varied and delicate experiments have actually been re- 

 peatedly made from the time of Fresnel to the present, to detect if pos- 

 sible an influence of heat on weight, but without result. Indications 

 sometimes observed have always been found to be due either to cur- 

 rents in the best approximate vacuums or to the molecular reactions 

 from unequal absorption. It is evident that if heat radiation could 

 exercise the slightest influence upon gravitative attraction, it would be 

 possible in many cases to iuterjpose a screen to its action, contrary to the 

 observed fact generalized in our fifth proposition. 



We are correctly reminded that " we only know the force of gravity 

 as between bodies such as they actually exist ;" but this knowledge 

 teaches us that the ratio ^ is invariable ; that therefore the exponent 2 

 of the radius or distance is integral, and is not a residual of an exponent 

 2+ n. Finally, we know that heat radiations require eight minutes to 

 pass from the sun to the earth. The inferences then that the conduct of 

 the " radiometer " affords any key to the problem of celestial mechanics, 

 or that it illustrates or suggests any analogous "agent acting con- 

 stantly according to certain laws" as " the cause of gravity," are evi- 

 dently unfounded and erroneous. " Nor must we forget that the more 

 rigidly we scrutinize our received theories, our routine explanations and 

 interpretation of nature, and the more frankly we admit their short- 

 comings, the greater will be our ultimate reward." § 



* This view has since been accepted by Mr. Crookes himself. 



t If any such action really existed, its effects would be just as certain and as disas- 

 trous if applied gradually as if applied suddenly, 

 t Quarterly Journal of Science, April, 1876, vol. vi, p. 254. $ Loco citaU, p. 256. 



S 18 



