TRANSACTIONS OF SECTION B. 355 
recently shown that the colour of metallic glasses and films is determined, not only 
by the absolute size of the metal particles, but also by the proportion of the total 
volume they occupy in the medium in which they are diffused. The results of 
Mr. Garnett’s calculations are in close agreement with a number of the observa- 
tions on the colour and microstructure of thin metal tilms which I had already 
recorded, and they appear to me to supply the explanation of much that had 
appeared puzzling before. My own observations lead me to think that the actual 
microscopic particles which are to be seen, and the larger of whichtcanfalso be 
measured, in films and solutions or suspensions, do not in any way represent the 
ultimate units of structure which are required by Mr. Garnett’s theory, but that 
these particles are aggregates of smaller units built up in more or less open forma- 
tion. 
That a relatively opaque substance like gold may be so attenuated that when 
disseminated in open formation it becomes transparent is contrary to all our 
associations with the same operation when performed on transparent substances 
like glass or crystalline salts. The familiar experiment of crushing a transparent 
crystal into a perfectly opaque powder would not prepare us for the effect of 
minute subdivision on the transparence of metals, _At first it might be supposed 
that this difference is due to the very rough and incomplete subdivision of the 
crystal by crushing ; but this is not the case, for the perfectly transparent oxide of 
magnesium may be obtained in a state of attenuation comparable with that of the 
gold, by allowing the smoke from burning magnesium to deposit on a glass plate. 
The film of oxide obtained in this way is found to be built up of particles quite as 
minute as those of which the gold films are composed, yet the opacity of the oxide 
film is relatively much greater. The minute particles of the dielectric, magnesium 
oxide, scatter and dissipate the light waves by repeated reflection and refraction, 
while the similar particles of the metallic conductor, gold, act as electrical 
resonators which pass on some of the light waves while reflecting others. Speci- 
mens of films of gold and silver and of magnesium oxide are exhibited on the 
table and on the lantern screen. When the metallic particles are in this state of 
open formation and relative transparence, it was found that the electrical con- 
ductivity of the films had completely disappeared. Films of this description were 
found to have a resistance of over 1,000,000 megohms as compared with only six 
ohms in the metallic reflecting condition. 
Molecules in the Solid State. 
My examination of gold films and surfaces has revealed the fact that during 
polishing the disturbed surface film behaves exactly like a liquid under the 
influence of surtace tension. At temperatures far below the melting-point mo- 
lecular movement takes place under mechanical disturbance, and the molecules 
tend to heap up in minute mounds or flattened droplets. These minute mounds 
are often so shallow that they can only be detected when the surface is illumi- 
nated by an intense, obliquely incident, beam of light. I have estimated that these 
minute mounds or spicules can be seen in this way in films which are not more 
than five to ten micro-millimetres in thickness. A film of this attenuation may 
contain so few as ten to twenty molecules in its thickness, 
is When moderately thin films of gold are supported on glass and heated at a 
temperature of 400°-50U°, they become translucent, and the forms assumed under 
the influence of surface tension can be readily seen by transmitted light. It was 
in this way that the beautiful but puzzling spicular appearance by obliquely 
reflected light was first explained as due to the granulation of the surface under 
the influence of surface tension. Photo-micrographs of these films are exhibited. 
Turning now to the mechanical properties ot metals, we find that gold has 
proved itself of great value in the investigation of some of these. It has long been 
recognised as the most malleable and ductile of the metals, whilst its chemical 
indilference tends to preserve it in a state of metallic purity throughout any pro- 
longea series of operations. ; 
The artificers in gold must very early have learned that its malleability and 
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