COLOURS IN METAL GLASSES AND IN METALLIC FILMS. 399 



indeed all gold glasses for which our condition is satisfied, should he much more red 

 than yellow. Presumably, therefore, they are still more yellow than green, and more 

 green than blue.* We should therefore expect the gold glasses F, G, H, which 



satisfy our condition, to be red, as in fact they are. 



The above values for d are certainly of the right order, but they may be somewhat 

 too large. If we had taken the value of>, called the Total Gold Content in Table II., 

 the corresponding values of d would have been only half those given above. 



It is to be remembered that manufacturers, in making gold ruby glass for " flashing" 

 on to clear glass, use much more gold. A common value for the total gold content is 

 about :3.10~ 5 . 



By means of equation (15) and Table I. we can in this way predict whether a gold, 

 silver, or copper glass for which v = I '56 will transmit more red or more yellow light, 

 and whether such a glass containing small spheres of "potassium-sodium" will 

 transmit more yellow or more blue. 



We thus find that when there are several metal spheres to a wave-length 



1 Q 



Silver glass transmits yellow (/3/A. yellow < /3/A red), 

 Copper ,, ,, red (/3/A. red < /3/A yellow), 



Gold (/3/X c/8/X ,. ), 



Potassium-sodium glass transmits blue (/3'A blue < /3/A yellow). 



Fromjihe values of /3 on Table 1., p. 3%, we see that for a silver glass to absorb as 

 much red light as a gold glass does yellow, /u, would have to be ^ 7 - 3 Aj-, or, roughly, 

 eight times as great for the silver glass as for the gold. And, since the values of /3 

 for yellow and red light are more nearly equal for silver than for gold, in order to 

 produce the same coloration there \vould have to be even more than eight times as 

 much silver (by volume) us gold. I am told that manufacturers put in ten times as 

 much silver by weight into a silver glass as they put gold into a gold glass. 



Again, the very large value of ft for yellow light in a potassium-sodium glass shows 

 that such a glass would absorb as much yellow light as a gold glass with only ^ of 

 the amount of metal excreted. 



Thus a very slight excretion of the potassium-sodium metal would give a very 

 strong blue or violet coloration. This probably explains the colouring of soda glass 

 by radium, the radiation causing the excretion of the metal. 



In order to test this hypothesis I asked Mr. F. SODDY, on 9th November, 1903, at 

 University College, London, to examine whether the emanation from radium was 

 capable of colouring quartz glass in which there could evidently be no possibility of 

 the excretion of metal: He stated that he and Professor RAMSAY, had abeady made 

 this experiment and had found no coloration. 

 "At my request Mr. SODDY then placed a small piece of -coburless gold glass in a 



* See Appendix. 



