420 MR. J. C. MAXWELL GAENETT ON COLOURS IN METAL GLASSES, ETC. 



specific gravities of CAREY LEA'S gold-coloured silver, C, and normal silver is 

 given by him to be 8'51/10'62 = '81. This strongly supports the theory 

 that allotropic silver is of the nature of the media we have discussed. 



(v.) CAREY LEA'S silvers were very brittle, but could be toughened by heating. 

 Further, his gold-coloured silver could be transformed into normal silver by 

 shaking ; and this transformation could be greatly impeded by packing the 

 gold-coloured silver in cotton wool. These properties suggest a discontinuous 

 structure for allotropic silver. 



(vi. ) If we might assume an absorption graph of UK for blue light, the fact that 

 if light is obliquely reflected from a film of " B " silver, then the yellow light 

 is polarised in the plane of incidence and the blue perpendicular to that plane 

 can, I think, be explained by our theory : but the proof is not yet complete. 



(vii.) The red colour exhibited by all the more dilute forms of the allotropic silver 

 is in accordance with the fact, exhibited by the graph, that HK is smaller for 

 red than for yellow light for small values of /x. 



j Ai'i'BNDix, added -28th July, 1904. Using the values of the refractive index 

 and absorption coefficient of gold for red (C), green (E), and blue light, as given by 

 RriJKNS (' Wied. Ann.,' 1889), the following values of the quantity /3/X, which 

 governs the absorption of the gold glass, have been calculated : 



(n. . . 

 I HK 



Golcl^j 



! /8/x . . 



The refractive index of the glass has been taken to be T56, as in Table II., from 

 which the values of /3 for red and for yellow have been copied. 



The colours, in the order of the degree in which they are transmitted by gold glass, 



therefore are 



Red, Yellow, Blue, Green. 



The corresponding order for silver as obtained by calculation is 



Yellow, Red, Green, Blue. 



The order's accord with observations on gold-ruby glasses and silver glasses respectively. 

 It will be seen that large particles of gold (diameter > O'l /A) in a gold glass would, 

 by reflecting out the red and yellow light, give the glass a blue colour by transmitted 

 light, and a brown turbidity by reflected light as in glasses D of Table I.] 



