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XXVI. The Scattering of Light by Air Molecules. 

 By R. W. Wood, Major U.S.R* 



A RECENT paper by Strutt (Proc. Roy. Soc. (last 

 number) 1918) on the scattering of light by sup- 

 posedly clean air makes it appear worth while to publish 

 the results of some experiments which I made on the same 

 subject in 1902, but did not publish at the time, as it was 

 found that they were spurious. The apparatus, method, and 

 results were identical with those of Strutt, in fact the 

 diagram of his apparatus mioht have been a drawing made 

 from the apparatus which I employed. This is merely a 

 coincidence of course, resulting from the fact that the 

 apparatus is the obvious one to use. 



In my work I employed a spark instead of an arc, wishing 

 to have available the shortest possible waves. Photographs 

 of the cone of scattered light appeared in air which had been 

 forced through long tubes filled with tightly packed cotton 

 and dried over phosphorus pentoxide. The cone was also 

 seen visually if the eyes were thoroughly rested in the 

 dark. 



This made me suspicious, and I varied the conditions under 

 which the experiment was made employing eye observation. 



It was soon found that if the spark was stopped and the 

 tube thoroughly washed out with the purified air, absolutely 

 no trace of cone of scattered light was visible on turning on 

 the spark. In about ten seconds, however, a trace of the 

 cone appeared, and after the spark had been in operation for 

 a minute it was well developed Interposition of a glass 

 plate prevented the formation of the cone, if I remember 

 correctly. This appeared to prove conclusively that the 

 ultra-violet light caused a precipitation of something from 

 the air, causing a slight cloud. 



Substitution of sulphuric acid for the phosphorus pent- 

 oxide only made matters worse, a dense cloud forming in ten 

 or fifteen seconds after starting the spark. I was unable to 

 secure air in which the light of the spark failed to develop 

 a visible fog, and consequently abandoned the experiments, 

 which were designed to test experimentally Lord Rayleigh's 

 theory of the blue sky. It would be well to try air vaporized 

 from the liquid using no drying agents or cotton. Some six 

 or seven years later some experiments were described by a 

 French physicist, whose name I do not recall at the moment, 

 which snowed similar effects ascribed to the formation of 

 nuclei by the ultra-violet light. 



* Communicated bv the Author. 



