558 REPORT— 1903. 



time. The results of these recent observations confirm and extend Faraday's con- 

 clusions, and it is helieved that they also supply an answer to certain questions 

 which he raised. 



The condition of the greatest opacity is found in metal films which are in a 

 state of strain, and the condition of greatest transparence is found in films which 

 have been relieved from strain by annealing. Contrary, therefore, to the generally 

 accepted idea, the metal in gold leaf is in its most opaque condition. 



Increase of transparence in metals is accompanied bj' diminution of reflecting 

 power, and vice versa. This eft'ect can be seen most distinctly in translucent films, 

 but it is also quite evident in the surface of the more massive forms of metal. 

 Films of gold and of platinum 200 fifi in thickness have been obtained which are 

 translucent and optically continuous. Films less than 10 /Lt/x in thickness have also 

 been made which appear to be equally continuous and are perfectly transparent. 

 The thinner films are practically without metallic reflecting power, while even in 

 the thicker films the reflection is distinctly inferior to that of gold leaf. 



The process of annealing has been watched on the surface of metal, and the 

 phenomena were found to be similar in kind to those which occur in films sup- 

 ported on glass or mica. lu surface films also the increase of transparence was 

 well marked, and the return of the metal to the more lustrous but less transparent 

 condition of burnishing was evident. 



From the study of the phenomena observed in cutting, polishing, burnishing, and 

 annealing I have been led to the conclusion that the disturbance caused by these 

 operations temporarily confers a degree of freedom upon the molecules of the 

 surface layer which enables them to act like a viscous fluid subject to the influence 

 of the molecular forces as they manifest themselves in surface tension. The 

 dimensions and the forms of the grooves, ridges, and granules on the surface give 

 a general indication that the layer afl'ected by this freedom and by the surface 

 tension is many molecules in depth. 



It appears probable that the granular structure of the surface is largely a 

 result of surface tension. A similar structure can readily be developed in a very 

 thin film of a viscous fluid. If a little oil is spread on a slip of glass and then 

 almost completely wiped off", so that it is barely visible to the unaided eye, a 

 granular film is produced which gives a well-marked spicular appearance by 

 obliquely reflected light. A film of varnish on a non-reflecting support shows 

 the same structure. A thin film of fuchsin on glass shows a structure and a play 

 of colour which might almost be mistaken for that of a feebly reflecting gold 

 film. Films of o.xide or sulphide on metal surfaces show the spicular appearance 

 very brilliantly. 



This surface granulation appears to be almost universal, and I have never 

 failed to produce it in any solid with which I have experimented. 



Granular or spicular structure seems to be closely associated with the deposi- 

 tion of solids from solution. It is seen in thin films of metal deposited either 

 chemically or electrolytically. In precipitates formed in very dilute solutions 

 there are three stages in the appearance of the solid : (1) spicules or spicular films 

 of extreme thinness, (2) granules, and (.3) crystals. Spicules may be formed 

 singly ; but they often result from the breaking up of the thin films which are 

 formed at the surface of contact of the two reagents. Their pedetic movements 

 lead to their agglomeration into granules which sink to the bottom of the con- 

 taining vessel, where they become centres of attraction to the moving spicules and 

 grow by their absorption. Till the granule has reached a certain size and mass it 

 shows no indication of crystalline form or structure, and its form remains, under 

 the control of surface tension, rounded and granular. When a certain size is 

 reached the crystallic force begins to assert itself and to overpower surface tension, 

 and the rounded form begins to develop faces and angles till finally a well- 

 developed crystal is produced. 



