Line Spectra of Elementary Bodies. 231 



■ I beg you will permit your letter and this to appear in the Pro- 

 ceedings. It will make my note more useful if you will. 



Believe me 



Very faithfully yours, 



J. Xobmjls- Locktee. 



March 23. — Joseph Halt on Hooker, C.B., President, in the Chair. 



The following paper was read : — 



" On the Force caused by the Communication of Heat between a 

 Surface and a Gas, and on a New Photometer." By Prof. Osborne 

 Reynolds. 



This paper contains an account of an experimental investigation 

 undertaken with a view to support, by absolute measurements, the 

 theoretical arguments by which the author endeavoured to prove the 

 existence of reactionary forces or "heat-reactions " whenever heat 

 is communicated from a surface to a gas, and vice versa, and the 

 connexion between these forces and the motion caused by heat and 

 light falling on bodies in vacuo. 



Since the publication of the author's paper on this subject in the 

 'Proceedings' for April 18 7-1, the correctness of his conclusions 

 with regard to the existence of these heat-reactions has not been 

 controverted or even questioned ; while Professors Tait and Dewar, 

 after an elaborate investigation, entirely confirm the author, not only 

 in these conclusions, but also in his view as regards the explanation 

 of Mr. Crookes's experiments. Mr. Crookes, however, appears 

 entirely to repudiate this explanation, arguing, — 



1. That he obtains his best results in vacua so perfect that there 

 is no air either to receive the heat or react on the surface. 



2. That the force is radiant in character. 



3. That light, as well as heat, produces the motion, which conse- 

 quently cannot be due to the heating of the surface. 



Having obtamed one of the beautiful little " light-mills " con- 

 structed by Dr. Greissler, of Bonn, the author was in a position to 

 make quantitative measurements of the effects produced and of 

 the force producing them. 



In the first place, with regard to the sufficiency of the residual 

 air to cause the motion. It was found that this air is, with the 

 exception of the friction of the pivot, which is found to be so 

 small as to be inappreciable, the sole cause of the resistance which 

 the mill experiences, of the liinit which is imposed on its speed 

 for each intensity of light, and of the rapidity with which it comes 

 to rest when the light is removed. The law of resistance, as deter- 

 mined by careful measurements, is found to agree perfectly with 

 the resistance which highly rarefied air would offer to its motion ; 

 and this law is distinctly special in its character, being propor- 

 tional to the velocity at low speeds, and gradually tending towards 

 the square of the velocity as the speed increases. 



Having established the fact that there is sufficient air in the mill 

 (and Mr. Crookes's behaves in the same manner as this mill) to 

 balance, by its resistance, the force which moves the mill, it is 

 argued that all question as to the sufficiency of the air to cause the 

 forces is removed. What the air can prevent it can cause. 



