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XXXIII. The Scatterinq of Light by Air Molecules. 

 By The Hon. R. J. Strutt, F.R.S.* 



PROF. R. W. WOODf has made some comments on my 

 paper on this subject { to which it seems desirable to 

 reply. I may say that at the time the paper was written 

 I was fully aware of the pitfall which Wood refers to. I was 

 working in the Cavendish Laboratory at the time that 

 C. T. R. Wilson made his experiments there on the preci- 

 pitation of clouds from moist air by ultra-violet light §, and 

 have always borne in mind that this might occur, even with 

 air that had been passed over phosphorus pentoxide : for 

 complete drying is known to be a slow process. 



On referring back to my paper, I see that I did not 

 adequately explain the precautions taken on this point : 

 thus Wood's demand for more evidence is quite justified, and 

 it only remains to meet it. m 



Many of my experiments, including some of the earliest, 

 have been made with a glass lens (a cheap plate-glass 

 lantern condenser), and the visual intensity of the scattered 

 light was not perceptibly less than with the quartz one, 

 though I did not attempt any strictly quantitative comparison. 

 I also found that a cell of quinine solution, which cuts out all 

 rays more refrangible than \4000, made no difference to 

 the visual intensity, though naturally it reduced the photo- 

 graphic intensity considerably. 



Another test often employed (and this is mentioned in my 

 published paper) is to have a rapid current of filtered air 

 going through the apparatus. This would prevent the 

 accumulation of fog, and should at any rate greatly reduce 

 the intensity of the scattered light, if due to fog ; but in fact 

 the intensity was the same as when the air was still. 



In later experiments in course of publication by the Royal 

 Society, I have worked with a variety of gases other than air, 

 and in some cases the formation of fog has proved troublesome. 

 There is not much uncertainty in practice as to whether a fog 

 has been formed or not, in any given case ; for in a series of 

 exposures, the intensity varies with time, as Wood remarks. 

 Frequently, too, a streakiness is observable in the photo- 

 graphed image of the scattered beam. I have gone into 

 these questions more fully in the paper referred to, which was 



* Communicated by the Author. 

 t Phil. Mag. vol. xxx vi. p. 272, Sept. 1918. 

 1 Proc. Rov. Soc. A. vol. xciv. p. 453 (1918). 

 § Phil. Trans. A. vol. cxcii. p. 412 (1899). 



