ON COLOURS IN METAL GLASSES, ETC. 285 



All these properties are shared by FARADAY'S " phosphorous " gold,* so that our 

 expectation is, so far, fulfilled. We are, in fact, perfectly familiar with " mere 

 aggregations of discontinuous material " which are optically continuous for example, 

 gold ruby glass. 



Many of the observations which CAREY LEA has recorded on the colours of his 

 silver films are in accordance with the expectation that these films, like BEILBY'S 

 gold films and FARADAY'S " phosphorous " gold, should behave according to the laws 

 (a), (6), and (c) stated above. But two difficulties arise in the way of this 

 accordance, for, in the first place CAREY LEA'S recorded observations do not 

 sufficiently distinguish between transmitted and reflected light. For example he 

 recordst that his freely precipitated silver dissolves to a blood-red colour, and 

 proceeds 



"When the substance is brushed over paper and dried rapidly it exhibits a beautiful succession of 

 colours. At the moment of applying it it appears blood red| ; when half dry it has a splendid blue colour 

 and lustrous metallic reflection;! when quite dry this metallic effect disappears and the colour is matt 

 blue."|| 



Lastly, in the case of the films discussed in 11 above, the colour depended on the 

 fact that the density of the film was less than that of the metal composing the film 

 when in its normal state ; but pressure increased the density to its normal value, at 

 the same time bringing out the normal colour, both by reflected and by transmitted 

 light, of the metal. And CAREY LEA'S silvers " show a lower specific gravity than 

 that of normal silver ; "1 and pressure "instantly converted gold-coloured allotropic 

 silver into normal silver."** 



We conclude from the above evidence that this silver was not " allotropic," but 

 consisted of normal silver in a finely divided state. 



14. HERMANN VOGEL'S Silver. 

 Before leaving the consideration of these discontinuous forms of silver, reference 



O 



must be made to a paper by HERMANN VoGEL,tt in which the author describes how 



* " The least touch of the finger removed the film of gold. . . . These films, though they are certainly 

 porous to gas .... have evident optical continuity " (FARADAY, loc. cit., p. 439). Of. also the facts that 

 films analogous to CABBY LEA'S did not conduct (BARUS and SCHNEIDER, loc. cit., p. 285), and that the 

 phosphorous films did not sensibly conduct electricity (FARADAY, loc. cit., p. 407). 



t 'Brit. Journ. Phot.,' March, 1901, p. 19. 



\ This is the colour by transmitted light when /* is fairly small. Cf. figs. 9 and 11. 



This is the reflected colour for values of p from zero to nearly - 8. Cf. fig. 13. 



|| Professor R. W. WOOD repeated this experiment, using glass instead of paper to support the silver 

 film. The metallic effect, then, does not disappear, but remains after the film has become quite dry. 

 Cf. above, p. 282. 



H 'Brit. Journ. Phot.,' March, 1901, p. 21. 

 ** 'Phil. Mag.,' vol. 31, p. 244, 1891. 

 tt 'Pogg. Ann.,' CXVIL, p. 316, 1861. 



