of Silver and of some other Metals to Light. 



490 



on the silver under the influence of light, it must be produced by the 

 jgency of gases or vapours contained — 



(i) In the silver itself. 



(ii) In the layer of air or condensed gases in immediate contact with 

 the plate. 



(iii) In the surrounding atmosphere. 



(iv) In the masks or coverings imder which the exposiu'es are made. 

 What the exact nature of the decomposition may be, and to which 



of these agencies it is attributable, is not easy to ascertain, nor does 

 the appearance of the visible image give much clue to it. From 

 the dull-grey colour of the exposed parts it seems, however, probable 

 that ox}^gen plays an important part in the action ; ])ut whether 

 the oxygen is drawn from the air or is disengaged from the metal 

 itself, or whether both actions take place with interactions of other 

 gases occluded in the metal itself or present in the atmosphere in 

 contact with its surface, there is nothing so far to definitely show. 

 It seems also probal)le that, as in many other photographic processes, 

 the presence of watery vapour is necessary to bring about the decom- 

 position. These points require fiu'ther investigation at a time of the 

 year when the light is bright and the effects can be observed under 

 the most favourable conditions. From the fact that in most of my 

 experiments the external air has been excluded by the outer glasses 

 of the pi'inting frames, we may, I think, conclude that the effects are 

 due more to the gases or vapours occluded in or attached to the silver 

 surface or to the screens, rather than to any oiuside atmospheric 

 influences. 



Oxygen found in Silver hij Grahain. — With regard to the presence of 

 oxygen in silver, Graham found that silver heated and allowed to cool 

 in oxygen could occlude 0'745 volume of oxygen, which was per- 

 manently fixed in the metal at all temperatures below an incipient red 

 heat. It did not tarnish the bright metallic surface of the silver or 

 produce any appearance suggestive of the oxidation of a metal. He 

 further showed that silver in the form of sponge can occlude 8 volumes 

 of oxygen without any visible tarnish. Silver appears to have a rela- 

 tion to oxygen similar to that exhibited by platinum, palladium, and 

 iron to hydrogen. Silver can also occlude variable quantities of 

 hydrogen, nitrogen, and carbonic acid.* 



Dumas' s Ohservations. — According to Dumas, f 1 kilo, of pure silver, 

 prepared by fusion with borax and nitre, was heated in a vacuum to a 

 temperature not exceeding a dull red, about 500° or 600° C. The 

 evolution of gas continued for about six hours, a.nd it was received 

 over mercury. The gas given off' was pure oxygen, amounting to 

 47 c.c. at 0°, and 760 mm. of pressure for 1 Idlo. of silver, and 

 * ' PHI. Trans.,' 1866, p. 434. 



t ' Comptes Eendus de I'Acad. France,' Tol. 86, 1878, p. 69. 



