of the Law of Reflexion of Gas Molecules. 303 



upper oblique half of the bulb with the exception of a narrow 

 circular zone just above the plane of the plate. 



This clear zone is extremely interesting. 



I at first attributed it to the circumstance that the dia- 

 meter of the bulb was small in comparison with that of the 

 reflecting surface. To test this hypothesis the experiment 

 was repeated with a larger bulb, and a deposit as shown in 

 fig. 3 was obtained, in which the cosine law and the random 

 direction of reflexion are well shown. Professor Max Mason, 

 of the University of Wisconsin, made a calculation of the 

 distribution of density in a bulb small in comparison with the 

 reflecting surface, which by the way is not an easy problem, 

 and found that the clear zone was not accounted for on this 

 supposition. I then repeated the experiment with the large 

 bulb, and found that, if the deposit was allowed to form for 

 fifteen or twenty minutes, the clear zone was quite as 

 marked as with the smaller bulb. It occurred to me that 

 the clear zone might have something to do with the structure 

 of the surface, irregularities of molecular size perhaps pre- 

 venting the nearly grazing reflexion. I tried a freshly split 

 surface of mica mounted on a glass plate, and obtained the 

 deposit shown in fig. 4, the clear zone being quite as con- 

 spicuous as with the glass surface. 



Dr. Wright, of the Geophysical Laboratory, informs me, 

 however, that some experiments which they have made, 

 suggested by the account of my experiments which I gave 

 at the Spring meeting of the National Academy, have yielded 

 quite remarkable results in the case of the reflexion from the 

 surfaces of certain crystal sections. An account of these 

 experiments will be published shortly. 



In the reflexion experiments the reflecting surface keeps 

 the temperature which it had before the immersion of the 

 apparatus in liquid air, as it is isolated in a non-conduct- 

 ing vacuum, and the only escape for the heat is through the 

 long thin stem which supports it. In some cases 1 heated 

 the bulb while it was still full of air, and in this way brought 

 the reflecting surface to a rather high temperature before 

 the exhaustion was commenced. The tubes were usually 

 provided with a lateral branch, joined on midway between 

 the two constrictions and bent up. The exhaustion was 

 effected through this tube, and it was found advantageous 

 to keep the pump in operation throughout the course of the 

 experiment. 



The results appear to show that the cosine law is approxi- 

 mately followed, for the density is greatest on the line of the 

 normal, falling off gradually as the angle increases. At 



