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XXIX. Experimental Determination of the Law of Reflexion 

 of Gas Molecules. By R. W. Wood, Professor of Experi- 

 mental Physics, Johns Hopkins University *. 



[Plate III.] 



VARIOUS phenomena connected with the flow of gases 

 have led us to assume that the gas molecule, on 

 collision with a solid wall, is reflected at an angle which is 

 quite independent of the angle of incidence, and that the 

 number reflected in any given direction is proportional to 

 the cosine of the angle which this direction makes with the 

 normal to the surface. This law is assumed by Knudsen in 

 his recent treatment of the kinetic theory, but so far as I 

 know it has never been verified directly by experiment. 

 Some eight or ten years ago I made some efforts to realize 

 what might be termed a one-dimensional gas by allowing the 

 gas to enter a long highly exhausted tube through a very 

 minute hole while in communication with a small bulb, 

 which was broken after the system had been for some time 

 in communication with the pump. It was assumed that the 

 first molecules to reach the far end of the tube would be 

 moving in parallel paths, and the electrical behaviour of such 

 a gas was the phenomenon studied, the tube being non-con- 

 ducting initially. As no very definite results were obtained, 

 it is not worth while to record the details of the experiment. 

 Attempts were also made to realize a one-dimensional gas by 

 vaporizing mercury in a tube, a portion of which was cooled 

 by solid carbonic acid, but these experiments gave also 

 negative results. The very interesting observations of 

 L. Dunoyer on the formation of sharply defined deposits of 

 metallic sodium in highly exhausted tubes, due to the pro- 

 jection of a flight of molecules down the tube, led me to 

 repeat my earlier experiments with mercury. 



M. Dunoyer had pointed out to me the necessity for employ- 

 ing a very perfect vacuum, and after one or two experiments 

 1 had no difficulty in observing the phenomenon with mer- 

 cury. In fact, mercury appears to be better adapted to 

 the production of the one-dimensional gas than sodium, for 

 the latter contains occluded hydrogen, and the last traces of 

 this can be removed only with great difficulty, if at all. 

 This gas impairs the vacuum very rapidly, and gives rise to 

 deposits with ill-defined edges as a result of molecular 

 collisions. 



* Communicated by the Author. 



