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with the silver bromide we only get crystals of the cubic system. 

 A microscopic examination of rijoened emulsion shows that the 

 particles vary in size from 0-4/a to about 10/x, but emulsions used in 

 practice do not often contain crystals larger than from 2-3/i. It is 

 difficult or almost impossible, to determine the shape of particles 

 smaller than 0-8/x in diameter, but all particles of this size and 

 larger are found to be forms of the cubic system, principally 

 hexagonal and triangular plate and tetrahedra. Since there is no 

 sharp break in the jjroperties of an emulsion at any particlar size 

 of particle, it is probable that even the smallest particles are crystalline. 

 There is no evidence of the existence of amorphous silver bron\ide. 

 When crystals of sUver chloride are grown from an ammoniacal 

 solution, cubes are formed, if gelatin is jjresent there seems to be a 

 tendency for the 1.1.1 faces of the crystal to develop. The simple 

 cube does not seem to be formed in the presence of gelatin. 



When light shines on the silver hahde a red coloration is j)roduced. 

 Luther showed that the halogen was liberated, and that the reaction 

 did not take j)lace if the pressure of halogen present exceeded a certain 

 equilibrium value depending on the intensity of the light. These, 

 coloured halides are often called photohalides, they may also be 

 prepared by subjecting a mixture of finely divided silver and silver 

 halide to a high pressure. It was formerly considered that these 

 jjhotohalides con.sisted of silver subhaUdes. The work of Sichling, 

 Lorenz und Eitel, and Lorenz und Hiege has, however, conclusively 

 jDroved that these photohalides are colloidal solutions of silver in the 

 halide. Sichling showed by E.M.F. measurements that if silver sub- 

 bromide existed at all in the halides, it was only to a very small extent, 

 and was only stable over a very short range of concentration. The 

 existence of colloidal silver m the photobromide of whatever composi- 

 tion was definitely established. Lorenz and his co-workers showed 

 that optically clear silver halides may be prepared by treating the 

 fused salt Vith the halogen. When these optically clear crystals are 

 exposed to light they darken, but remain at first optically clear, later 

 the surface at which the beam enters becomes brown and the particles 

 become visible in the ultra-microscope. The particles grow rapidly 

 in the light, and wUl continue to grow if they are removed from the 

 light and heated to 350° C. Heating without previous exposure to 

 light does not produce these particles. The growth of these particles 

 is accompanied by a diminution of coloration m the immediate neigh- 

 bourhood. The eftect is evidently due to the separation of colloidal 

 silver in the metallic form. The analogy between these fogs and 

 the metallic fogs formed in fused salts seems to be complete. It is 

 probable that the latent image in the photographic plate consists of 

 a colloidal solution of silver in the halide, and is the first stage of 

 the formation of the photohaHde. Many theories of the latent image 

 have been put forward, among which may be mentioned that of 

 H. S. Allen, who has suggested that the latent image is due to the 

 loss of an electron by a molecule of the silver halide and the electron 

 remains embedded in the gelatin. Under certain circumstances the 

 electron may get back and the latent image may be destroyed. This, 

 and many other theories have never been put to the proof. Some- 



