444 



Colours in Metal Glasses and Films. 



[Apr. 19, 



containing metal spherules, so that the average distance between two 

 neighbouring spheres is considerably less than a wave-length of light, 

 is next undertaken. It is shown that every such medium has a 

 perfectly definite colour by transmitted light, depending only on the 

 optical constants of the metal of which the spheres are made, on 

 the refractive index of the substance in which they are embedded, and 

 on the quantity of metal, but not on the size or distance apart of the 

 spheres. 



The intensity of the absorption of light of each colour is pro- 

 portioned to /x, the volume of metal per unit volume of medium. 

 It is calculated, by means of the metal constants given by Drude,* 

 that, with glass of refractive index 1*56, gold glass is more red than 

 yellow, silver glass a little more yellow than red, copper glass con- 

 siderably more red than yellow, and " potassium-sodium " glass much 

 more blue than yellow, provided always that the average distance 

 between two neighbouring particles of metal in the glass be consider- 

 ably less than one wave-length ; in which case, as stated above, the 

 particles must be spherical. Metal glasses for which this provision is 

 satisfied will be called "regular.'' 



It is next proved that the presence of metal spheres accounts for the 

 optical properties of regular gold ruby glass, and that the irregularities 

 in the effects of colour and polarisation, sometimes exhibited by gold 

 glasses, are due either to excessive distance between adjacent gold 

 particles or to excessive size of such particles — the latter, however, 

 involving the former. 



Experiments are described, proving that this regular colour can be 

 produced in a colourless metal glass, containing the metal in solution 

 (which is the sta f e in the manufacture of gold or copper ruby glass 

 before the second heating) by the ^-radiation from radium. Thus, 

 a piece of clear gold glass and a piece of clear soda glass were exposed 

 to the emanation for two days, when the gold glass had acquired an 

 unmistakable pink tint, while the soda glass had turned an intense 

 blue-violet. 



In the second part of the paper, the optical properties of media 

 built up out of metal spheres as before, but now so that the volume of 

 metal may have any value between zero and unity, instead of 

 remaining very small, as in metal glasses, are investigated. The 

 changes of colour, and the final change to almost complete trans- 

 parency, observed by Mr. G. T. Beilbyf in gold and silver films, are 

 accounted for. Explanations are also given of the changes of colour 

 on heating, observed by Professor E. W. Woodj, in potassium and 

 sodium films deposited on the insides of exhausted glass bulbs. The 



* ' Phys. Zeitschrift,' January, 1900. 

 f ' Roy. Soc. Proc.,' vol. 72, 1903, p. 226. 

 X ' Phil. Mag.,' 1902, p. 396. 



