GOLD IK SCIENCE AND IN INDUSTRY. 221 



the size of the particles in different specimens was found to vary 

 from 4 to 701 millionths of a millimeter. No relation was found to 

 hold between the color of the ])articles and their ahsohite size. This 

 conclusion is in direct contradiction of Faraday's belief, already 

 referred to. Mr. J. Maxwell Garnett has recently shown that the 

 color 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 occu})y in the medium in which they are dilfused. 

 The results of Mr. Garnett's calculations are in close agreement with 

 n number of the observations on the color and microstructure of 

 thin metal films A>hich I had already recorded, and they appear to 

 me to su})ply the exi)lanation of nnich that had appeared j)uzzling 

 before. ]\Iy own ol)servations lead me to think that the actual micro- 

 scopic })articles which are to be seen, and the larger of wliich can 

 also be measured, in films and solutions or suspensions, do not in any 

 way represent the ultimate units of structure which are re(iuired 

 by ^Ir. Garnett's theory, but that these particles are aggregates of 

 smallei* units built up in more or less open formation. 



That a relatively o]:)aque sul)stance, like gold, may be so attenuated 

 that when disseminated in open formation it becomes transparent is 

 contrary to all our associations Avith the same operation when per- 

 formed on transparent substances like glass or cr^^stalline 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 crj'stal 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 wav 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 b}'^ repeated reflection 

 and refraction, while the similar joarticles of the metallic conductor, 

 gold, act as electrical resonators which pass on some of the light 

 waves while reflecting others. vSpecimens of films of gold and silver 

 and of magnesium oxide are exhil)ited on the table and on the lan- 

 tern screen. "When the metallic particles are in this state of open 

 formation and relative transparence, it was found that the electrical 

 conductivity 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 6 ohms in the metallic reflecting 

 condition. 



