226 Mr. G. T. Beilby. The Effects of [June 4, 



" The Effects of Heat and of Solvents on Thin Films of Metal." 

 By G. T. BEILBY. Communicated by F. H. NEVILLE, F.E.S. 

 Eeceived June 4, Kead June 18, 1903. 



[PLATES 1113.] 



In the Bakerian Lecture of 1857,* on " Experimental Relations of 

 Gold and other Metals to Light," Faraday described a series of experi- 

 ments which were designed to throw light on the structure and 

 behaviour of metals in their most attenuated forms. Probably the 

 most remarkable of these experiments were those in which leaves and 

 films of gold and silver supported on glass were changed by a tem- 

 perature much below the melting point of the metal from a moderate 

 translucence to clear transparence and from high metallic reflecting 

 power to comparative deadness. 



These remarkable experiments seem practically to have dropped out 

 of sight during the past 45 years for, so far, I have found no reference 

 to this particular phenomenon in the papers of more recent workers on 

 the reflecting and absorbing powers of thin metal films, and many 

 physicists to whom I have shown these Faraday films have received 

 them as a novelty. 



The significance and explanation of the change produced by heat is 

 discussed by Faraday at several stages of the lecture. Two alternative 

 explanations are suggested by him. Under the first, it is supposed 

 that the heating may act by gathering the metal into small aggregates, 

 thus leaving the surface like a grating through which light can freely 

 pass. Under the second, the leaf or film is supposed to be made up 

 of scale-like films, the effect of heating being to open these up like the 

 louvres of a blind, thereby allowing the light to pass freely between 

 them. A third suggestion may be read into some of Faraday's 

 remarks, namely, that metal in the non-reflecting transparent condition 

 is an allotropic modification of the common or reflecting form. 



While studying the appearance and structure of surface films on 

 metals in their more massive forms,! it occurred to me that consider- 

 able light might be thrown on that subject by a parallel study of the 

 behaviour of translucent films supported on glass, as these present the 

 great advantage for microscopical examination that they permit of the 

 use of transmitted light. 



The result of this parallel study has been to show that both aggre- 

 gation and film formation come into play when metal leaves and films 

 are heated, but that an intrinsic transparence exists altogether apart 

 from these phenomena. 



* ' Phil. Trans.,' 1857, p. 145. 



f ' British Association Report,' 1901, p. 604. 



