Kinetic Theories of Gravitation/Crookes, 1873

From some very ingenious and interesting experiments made by William Crookes, in 1873, with light disks attached to a delicate torsionbalance and hermetically sealed within a nearly perfect vacuum, in which experiments the radiation of heat was observed to exert a repulsive 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 quasivacuum of space we have the radial repulsive force, possessing successive propagation, required to account for the changes of form in the lighter matter of comets and nebulae ; . . . . but until we measure the force more exactly, we shall be unable to say how much influence it may have in keeping the heavenly bodies at their respective distances. 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,' 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 impulsion of radiation, but solely by the differences of heat-absorption by [273] the two sides of the disks and the reactions of the residual air of the exhausted chamber.

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; — sufficient to knock the earth out of its orbit if it came upon it suddenly. It may be said that a force like this must alter our ordinary ideas of gravitation ; but it must be remembered 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 generalizations 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 induction can therefore be arraigned only by a conflicting fact of observation or experiment. Varied and delicate experiments have actually been repeatedly made from the time of Fresnel to the present, to detect if possible an influence of heat on weight, but without result. Indications sometimes observed have always been found to be due either to currents 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 interpose 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 m/r² 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 constantly according to certain laws" as " the cause of gravity," are evidently 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 shortcomings, the greater will be our ultimate reward." [274]