﻿450 Prof. Threlfall on the Scattering 



silver nitrate. In tins case I did not succeed in satisfying 

 myself that chlorides were sufficiently eliminated. The sky, 

 whatever it was composed of, behaved just as before. 



o. Iron. — A properly reduced solution of ferrous ammonium 

 sulphate was obtained and mixed with phosphorus-water as 

 before. The solution was more or less preserved from oxida- 

 tion by covering it with a layer of " albolene " oil. I am not 

 sure that an appreciable increase of the sky took place on this 

 occasion by the action of the phosphorus : what sky there was 

 behaved as before. 



The above experiments were repeated, a piece of phosphorus 

 being left in the liquid : but though the skies became rather 

 stronger, no differences could be detected. These experiments 

 suffer from the uncertainty that the skies might have been 

 formed by particles separated direct from the phosphorus. 



Assuming that Prof. Thomson's views are correct, these 

 experiments must, 1 think, be taken as showing that at all 

 events gold in particles fine enough to scatter light behaves 

 as an insulator. At all events, the phenomena are consistent 

 with the great transparency of gold-leaf. 



No difference was observed between the scattering of plane 

 waves, and the scattering from a pencil of rays converging 

 from the condenser of the lantern. 



Returning to the gold solutions, I have no hesitation in 

 saying that they afford the simplest and most beautiful 

 illustration of the colours produced by fine particles. It 

 is worthy of note that the colours are so brilliant, even if we 

 allow for the manner in which the colour of the scattered 

 light depends on the size of the particles. 



The apparent perfect transparency of the more dilute 

 solutions, when considered in connexion with the comparative 

 sharpness of the absorption spectrum, must indicate a uni- 

 formity in the dimensions of the particles which is, so far as I 

 know, unique. 



Judging from the appearance of the spectrum only, espe- 

 cially in the case of the stronger solutions — i. e., those that 

 just do not settle by themselves — I think that perhaps an 

 excellent light for photographic purposes might be obtained 

 by using these skies as light- filters. 



Supplementary Note. 



After concluding the above experiments it appeared to me 

 that the only metal for which the evidence I had to offer was 

 satisfactory was gold. I therefore made the following expe- 

 riments with the object of observing the action of a u sky " 



