ON COLOURS IN METAL GLASSES, ETC. 281 



colour, which was frequently resolvable into a mixture of green and amethystine 

 striae. These colours would be shown by an amorphous or granular film for which 

 the density was in parts as low as p. = 7. Moreover, such a structure agrees with the 

 fact that " the films did not sensibly conduct electricity " and that " the films cannot 

 be regarded as continuous."* FARADAY further statest that, though they are certainly 

 porous to gas and to water-vapour, the films have evident optical continuity. 



Heating diminished the conducting power and changed the colour to amethyst or 

 ruby, just as with BEILBY'S films, pressure, which we should expect to increase the 

 density of the film, changed the transmitted colour to green and increased the 

 reflecting power ; and these are precisely the changes which would, according to 

 calculation, accompany an increase in /A to the neighbourhood of unity in the case of 

 an amorphous or granular film. 



Closely allied to these phosphorous films are the deposits of gold on glass which 

 FARAD A Yj obtained " by deflagrating a gold wire by explosions of a Leyden battery." 

 " There is no reason to doubt that these deposits consisted of metallic gold in a state 

 of extreme division." This method of preparing these deposits is similar to BREDIG'S 

 method of obtaining suspensions of gold in water ; it is, therefore, to be expected that 

 the deposits consist of small spheres of gold .together with some large crystallites. 

 The films were so discontinuous as to be unable to conduct electricity ;|| but they were 

 such as to present an optical continuity.il FARADAY sums up their colour changes as 

 follows : 



" Fine gold particles, loosely deposited, can in one state transmit light of a Hue-grey colour [/j. = about 8], 

 or can by heat be made to transmit light of a ruby colour [/* < 7], or can by pressure from either of the 

 former states be made to transmit light of a green colour,** all these changes being due to modifications 

 Of gold as gold and independent of the presence of the bodies on which for the time the gold is supported." 



It appears, therefore, that the conclusions (a), (b), (c), arrived at on p. 280 for 

 BEILBY'S films, are also applicable to FARADAY'S "phosphorous" films and to 

 FARADAY'S " deflagration " films. 



One more experiment of FARAD AY'stt on coloured gold deposits remains to be 

 noticed. When a drop of solution of chloride of gold is evaporated in a watch-glass 

 until the gold is reduced, a portion of the gold is generally found to have been carried 

 by the vapour on to the neighbouring part of the glass. This part has the ruby tint ; 

 and we have seen that a ruby tint is characteristic of the light transmitted by 



* Loc. cit., p. 407. 



t Loc. cit., p. 439. 



J Loc. cit., p. 401. 



Cf. above, p. 252, and footnote, p. 255. 



|| Loc. cit., p. 402. 



U Loc. cit., p. 439. 



** Probably /A = 1 nearly ; but see fourth footnote on p. 280, above, 

 ft Loc. cit., p. 428. 

 TOL. COV. A. 2 O 



