284 MR- J - C. MAXWELL GARNETT 



for the allotropy of silver in the form in which he prepared the metal. We proceed 

 to examine these arguments. 



In the first place, then, all CAREY LEA'S silver films were prepared from silver 

 suspensions. He claims that these suspensions were "true solutions," and that the 

 ability of the silver to remain in solution in water was evidence that the molecules of 

 the silver in question differed from those of normal silver, or, in other words, the 

 silver was in an allotropic form. We are now, however, familiar with the fact that 

 particles of normal silver, as of many other metals, are able, in consequence of mutual 

 electrostatic repulsions,* to keep themselves in suspension in quite pure water. 

 Again, we have seen that, when a silver solution is prepared by BREDIG'S method, its 

 refractive index is that which is possessed by a suspension of small spheres, but not 

 of molecules, of silver in water, f and in the same case there is a strong absorption 

 band at exactly that point of the spectrum at which small spheres, but not molecules, 

 of silver in water would produce a maximum ;J so that in this case the greater part 

 of the silver is certainly present in the form of small spheres. Further, if, when 

 prepared by deflagration, silver in suspension in water takes the small sphere form, it 

 is primd facie probable that it does the same when obtained by CAREY LEA'S 

 method, and this probability is increased by the fact that CAREY LEA'S silver 

 suspensions exhibited the same red, yellowish-red, and yellow colours which are 

 shown by BREDIG'S suspensions of different densities. 



We conclude that CAREY LEA'S " solutions of allotropic silver " consisted of small 

 spheres of normal silver in suspension. || 



We should therefore expect that the films obtained by CAREY LEA would be 

 similar in constitution and behaviour to BEILBY'S " gold paint " filmsH and to 

 FARADAY'S phosphorous films.** This leads us to CAREY LEA'S second argument for 

 the allotropy of his silver ; he states : 



"The brittleness of the substances B and C [blue and gold coloured respectively, by reflected light], the 

 facility with which they can be reduced to the finest powder makes a striking point of difference between 

 allotropic and normal silver. It is probable that normal silver, precipitated in fine powder and set aside 

 moist to dry gradually may cohere into brittle lumps, but there would be mere aggregations of discontinuous 

 material. With allotropic silver the case is very different, the particles dry in optical contact with each 

 other, the surfaces are brilliant, and the material evidently continuous. That this should be brittle 

 indicates a totally different state of molecular constitution from that of normal silver." ft 



* See footnote p. 253 above, 

 t See above, p. 258. 

 t See above, p. 257. 

 Above p. 259. 



Cf. also the fact that the silver in a silver-stained glass is in the form of small spheres. 

 || Cf. also evidence given on p. 259 above. 

 H See above, p. 279. 



** See above, p. 281. This expectation is verified by a further examination of WOOD'S films. See note 

 above p. 282. 



tt 'Brit. Jour. Phot.,' March 1901, p. 21. 



