282 MR. -I. C.' MAXWELL GARNETT 



amorphous or granular gold, the density of which is in the neighbourhood of '6 of 

 that of normal gold. 



The similarity of this method of preparing a metallic film with \L < 1 to that by 

 which Professor R. W. WOOD prepared the sodium and potassium films, described in 

 12 of the former communication,* is deserving of notice, and, from a different 

 standpoint, tends to confirm the view there expressed as to the physical nature and 

 structure of Professor WOOD'S films. 



The conclusions (a), (b), (c) arrived at above (p. 280) as to the effect of heat 

 and pressure on metallic films do not apply only to gold, as the following observations 

 on silver films show. FARADAY! obtained silver films by reducing silver from a 

 solution of the nitrate. The thinner parts of these films transmitted light of a 

 " warm brown or sepia tint [//, < '8]. Pressure brought out the full metallic lustre 

 and converts the colour from brown (ju, < '8] to blue [p. > '8]." The behaviour ot 

 these films corresponds to that of the gold films obtained with phosphorus. Again, 

 ft. W. WOOD| prepared films by chemically depositing silver on glass. These films, 

 as originally prepared, show the same reddish-brown colour by transmitted light, and 

 have a good blue-green reflection. It has been shown above that both these colours 

 are characteristic of amorphous or granular silver, for which p. is appreciably less 

 than - 8. These films showed no electrical conductivity ; so that, as in the case of 

 BEILBY'S gold films, || the evidence of a loose structure afforded by the colours 

 exhibited is confirmed by the evidence from conductivity. 



12. The Exceptional Case of Beaten Metal Leaf. 



There is one class of metallic film which, when heated, does not exhibit the colour 

 changes that, according to our calculations, correspond to a gradual diminution in 

 the density of the film. To such films the conclusions (a), (b), (c) of p. 280 do not 

 directly apply. Instead of being obtained from finely divided metal by chemical 

 deposition, deflagration, &c., the films in question are prepared by beating sheets of 

 the solid metal into thin leaves. 



FARADAY IF observed that heat caused gold leaf to lose its olive-green colour and 

 silver leaf to lose its deep blue colour, the films at the same time becoming more 



* Loc. cit., p. 412. 



t Loc. cit., p. 409. 



| 'Phil. Mag.,' August, 1903. The silver was prepared by the method of CAREY LEA (' Amer. Journ. 

 of Sc.,' 1889). A further memoir on WOOD'S silver films is now in course of preparation. 



Of. BARUS and SCHNEIDER, ' Zeitschr. f. Phys. Chem.,' VIII., p. 285, 1891, who attempted to 

 measure the conductivity of a silver film prepared by CAREY LEA'S method, and found that, so soon as a 

 drop of the silver suspension dried, so that the charged particles of silver could no longer move about, the 

 conductivity of the drop vanished. 



|| See above, p. 280. 



f Loc. cit., p. 395 et seq. 



