392 MR. J. C. MAXWELL GARNETT ON 



of the films of metal deposited from solutions (' Proc. Roy. Soc.,' vol. 72, 1903, 

 p. 223). 



In the manufacture of gold and copper ruby glasses and of silver glass, the gold or 

 copper or silver is mixed with the other ingredients of the glass before the first firing. 

 If, when the glass is formed in the furnace, the whole be quickly cooled, the glass with 

 the metal in it is colourless and exactly resembles clear glass. I have had in my 

 possession several pieces of such clear gold glass, and some of clear silver glass. One 

 of the former was used in an experiment with the emanation from radium, to be 

 described later. 



In this clear glass the gold or silver is probably in solution in the glass. But 

 when the glass is re-heated the metal "crystallises" out of solution, or, as we shall 

 say, is " excreted " from the glass and appears in the small particles observed by 

 SIEDENTOPF and ZSIGMONDY. These particles of metal, as we shall show, account for 

 the colour of the glass. 



I have seen a piece of copper glass which was allowed to cool down slowly in the 

 glass pot along with the furnace, taking a week or more in the process. The glass 

 formed a dark brown, nearly opaque, mass with minute crystals of bright, shining 

 copper scattered throughout its substance, the crystals being large enough to be easily 

 distinguishable with the naked eye, while the appearance of the whole mass somewhat 

 resembled that of the well-known African stone, aventurine. 



It is suggested that the second heating, without melting the glass, confers sufficient 

 freedom on the molecules of the glass to enable the forces of surface tension to exert 

 themselves in bringing the molecules of the metal, which have been distributed 

 amongst those of the glass, together into heaps, the phenomenon being similar to that 

 exhibited when a metal film is heated to 300 or 400 without being melted, when, 

 as will be described later, the metal forms itself into minute granules, which, 

 in the light of what we have proved for the particles in gold glass, must be 

 spheres or spheroids with axes normal to the film. The latter form is possible 

 for the films of metal, though not for the metal in the gold glass, because a 

 thin film, as opposed to a piece of glass, is not subjected. to similar conditions in all 

 directions. 



4. We have thus to consider the problem of light traversing a medium containing 

 many small metal spheres to a wave-length of light. 



It has been seen ( 2) that a small metal sphere produces in all surrounding space 

 the same effect as would be produced by a Hertzian doublet placed at its centre. 

 We may therefore imagine the spheres replaced by such electric doublets and thus 

 avoid considering their finite size. 



Let the average (for a large number of doublets) moment of a doublet be, at time t, 



Then if there be 3? spheres per unit volume, the polarisation of the medium will be 

 f (t) = 5J?f (t). If E', due to the regular force E together with forces due to the 



