COLOURS IN METAL GLASSES AND IN METALLIC FILMS. 415 



11. In a paper on "The Effects of Heat and of Solvents on Thin Films of 

 Metal," 'Boy. Soc. Proc.,' vol. 72, 1903, p. 226, Mr. G. T. BEILBY gives an account ot 

 some experiments on the behaviour of gold and silver films when heated to tempera- 

 tures far below their melting points. He suggests that at such temperatures 

 sufficient freedom is conferred on the molecules by the heating to enable them to 

 behave as the molecules of the liquid metal would do, and to arrange themselves 

 under the influence of surface tension either in films or in drop-like granular 

 forms. 



We have already shown, when dealing with the colours in metal glasses, how the 

 small particles of metal excrete themselves from the glass into spherical forms. 



Mr. BEILBY records that the resistance of silver and gold films increased, on 

 annealing, from a few (0'2 up to 50) ohms up to many thousands of megohms. This, 

 of course, strongly supports the theory that the metal breaks up under surface 

 tension into minute granules. Professor WOOD observed no conductivity in his films 

 as originally deposited. Mr. BEILBY further states that in one of the gold films there 

 appeared to be a considerable depth of granules, and Professor WOOD records absence 

 of conductivity in a film in which granules appeared in contact with and piled upon 

 top of one another. These observations support our hypothesis its to the structure 

 of the films, although the granules observed may have been larger than those which 

 are effective in producing the colour phenomena which we are to investigate. 



Let us now see whether our hypothesis as to the structure of the films is in 

 agreement with the colour effects observed by Mr. BEILBY and Professor WOOD. 



First, then, consider a very thin film of gold. 



According to the result given in equation (24) the diminution in intensity of 

 light of wave-length X is, for such a film, iird/K. n~K. 



From the graphs of n~ for yellow and for red, it is seen that for the solid metal for 

 which /A = 1, i.e., before annealing, /r/c/A is less for red than for yellow, and this is 

 true from p. = I nearly down to p. "J. Thus, a very thin leaf of gold should not 

 show the green colour distinctive of gold leaf, but the red colour should predominate 

 over the yellow. The arbitrary graph for II'-K for blue would, if correct, show that 

 blue should predominate over either yellow or red. 



'The colour of a very thin film of gold leaf would, therefore, be chiefly blue, less 

 red, and least yellow, i.e., blue-purple, and this is the colour observed by Mr. BEILBY 

 in the thinnest piece of gold leaf he possessed (loc. cit., p. 227). 



It should be noticed that it has been proved that a very thin film will let through 

 more red than yellow light, and that it, therefore, will not exhibit the green colour of 

 gold leaf. It has only been stated that it seems probable that it will let through 

 more blue than either. 



We now suppose that when the film is being annealed, surface tension acts and 

 causes the gold to form into spherical drops, many to a wave-length, but of quite 

 varying sizes. Thus /ot, the volume of metal in a unit volume of the film, 



