478 Mr. E. H. Cook on the 



The Radiometer. 



Five or six years ago, when Mr. Crookes announced his 

 important discovery, it was thought that the death-blow had 

 been given to the undulatory theory of light, and the emis- 

 sion theory of Newton established in its stead. Bennett (Phil. 

 Trans. 1792) had previously tried, by concentrating the light 

 upon a balance supported by a single spider's web, to produce 

 motion, but without success ; no motion could be produced 

 by means of the light-ray. But in the radiometer apparently 

 continuous rotation of the fly was produced by the action of 

 the light-rays. The inventor himself, whilst carefully guard- 

 ing himself against pinning his faith to any theory, thus 

 speaks of his discovery at a time when the correct theory 

 of the action had not been propounded: — " But light, in some 

 respects the highest of the powers of Nature, has not been 

 hitherto found capable of direct conversion into motion ; and 

 such an exception cannot but be regarded as a singular ano- 

 maly. This anomaly, the researches which I am about to bring 

 before you, have now removed ; and, like the other forms of 

 force, light is found to be capable of direct conversion into 

 motion, and of being (like heat, electricity, magnetism, sound, 

 gravitation, and chemical action) mOst delicately and accu- 

 rately measured by the amount of motion thus produced." 

 (Lecture at the Royal Institution, Friday evening, Feb. 11th, 

 1876.) Shortly after the lecture in which these sentences ap- 

 peared was delivered, Mr. G. J. Stoney published his expia- 

 tion of the action of these instruments in two' papers which 

 appeared in the Phil. Mag. for March and April 1876. After 

 subjecting this theory to the most careful examination, it was 

 finally accepted by Mr. Crookes as giving a complete and 

 satisfactory explanation of the matter. Viewed in the light 

 thus thrown upon it, it will be seen that the rotation is not 

 due to the action of the light-waves as such, but that the 

 unequal heating of the disk and glass envelope gives rise to 

 the production of streams of molecules, which, reacting on 

 the vanes, produce rotation. The full investigation of this 

 theory will be found in the two papers by Mr. Stoney before 

 referred to, and also in the following: — Two additional papers 

 by Mr. Stoney — one " On Penetration" (Phil. Mag. December 

 1877), and one " On Polarization Stress in Gases " (Phil. 

 Mag. December 1878); and a paper by Mr. G.. F. Fitzgerald, 

 " On the Mechanical Theory of Crookes's Force" (Phil. Mag. 

 January 1879). An account of the rigorous experimental 

 tests to which Mr. Crookes subjected it will be be found in the 

 <Proc. Royal Society/ November 16, 1876, and April 26, 



