THE RADIOMETER. 
165 
It is perhaps not too much to say that in this instrument 
we have one of the grandest and purest demonstrations of the 
perpetual movement, the ceaseless flux and reflux, which is 
eternally going on in nature; an idea stated by the Greek 
philosopher in the axiom pel n rdvra, reappearing in the vortices 
of Descartes, and of late testified to by many and various ex- 
perimental acquisitions. Even in a logical point of view, con- 
siderable interest attaches to the mental process involved in the 
development of the radiometer. The fact of repulsion being 
given, and its explanation postulated, a rapid method of exclu- 
sion was needed, by means of which the cruder and less con- 
sistent hypotheses could be filtered off. To this the inventor 
immediately and almost instinctively applied himself ; nor would 
it be difficult to trace in successive modifications which the 
instrument has undergone the tabula prcesentice , the tabula 
absentice in proximo , the tabula graduum , and the other tests 
which in Bacon’s “Novum Organon” conduce 66 to a true and legi- 
timate induction, the very key of interpretation.” An attempt 
will be made in the following pages to show that “ after an 
exclusion correctly effected, an affirmative form will remain as 
the residuum, solid, true, and well defined, whilst all volatile 
opinions go off in smoke.” * 
For this purpose it will be desirable to take a brief survey, 
in chronological order, of the various steps in the evolution of 
the last and most complete form of the instrument, the otheo- 
meter, as they occur in the proceedings of the Royal Society, 
the “ Philosophical Magazine,” and in one or two foreign 
periodicals. 
On December 11, 1873, a paper was read by Mr. Crookes, 
which had been received in the preceding August, of which the 
title was, “ On the Action of Heat on Gravitating Masses.” 
The author therein stated that the experiments to be recorded 
arose from observations made when using the vacuum balance, 
in determining the atomic weight of thallium, for weighing 
substances of a higher temperature than the surrounding air 
and weights. There appeared to be a diminution of the force 
of gravitation, the experiments being instituted in order to 
render the action more sensible, and to eliminate sources of 
error. 
The preamble furnishes a historical resume of vague hints 
on the subject given by previous observers. The Rev. A. 
Bennet, in 1792, recorded the fact that a light substance deli- 
cately suspended in air was attracted by warm bodies; this 
he ascribed to air currents. Laplace spoke of the repulsive 
force of heat; Libri had made experiments, unsuccessfully 
repeated by Baden Powell; Fresnel obtained contradictory 
* Novum Organon, Book II. par. xvi. 
