388 Prof. O. Reynolds on the Force caused by Mar. 23, 



V. "Preliminary Report to Professor Wyville Thomson, F.R.S., 

 Director of the Civilian Scientific Staff, on Work (Chemical 

 and Geological) done on board H.M.S. ' Challenger.'' " By 

 J. Y. Buchanan, Chemist and Physicist to the Expedition. 

 (Published by permission of the Lords of the Admiralty.) 

 Received March 7, 1876. 



[These Eeports will appear in a subsequent Number of Proceedings.] 



March 23, 1876. 



Dr. J. DALTON HOOKER, C.B., President, in the Chair. 



The Presents received were laid on the table, and thanks ordered for 

 thein. 



The following Papers were read : — 



I. " On the Force caused by the Communication of Heat between 

 a Surface and a Gas, and on a New Photometer/'' By 

 Prof. Osborne Reynolds. Communicated by B. Stewart, 

 F.R.S., Professor of Natural Philosophy in Owens College, 

 Manchester. Received February 24, 1876. 



(Abstract.) 



This paper contains an account of an experimental investigation under- 

 taken 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 1874, 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 experi- 

 ments. Mr. Crookes, however, appears entirely to repudiate this ex- 

 planation, 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 consequently 

 cannot be due to the heating of the surface. 



