302 Actinism and Magnetism. [July, 



mates of silver and iodine are farther asunder, in the case of 

 those which have been exposed to the light, than they are 

 in the case of those which have not been thus stimulated 

 into vibration. Accordingly, when a developer is applied, 

 even a very long time after exposure, decomposition ensues 

 and silver is precipitated. 



When ozone is applied after exposure and before a deve- 

 loper, it prevents the action of the latter. For the attrac- 

 tion of ozone for silver is powerful, and uniting itself to that of 

 the iodine, it prevents the silver from being reduced to the 

 metallic condition by the developer. Or perhaps it may 

 neutralise the action of the latter, by supplying it with 

 something which it prefers to iodine. The vapours of 

 chlorine, bromine, fluorine, or iodine, applied after exposure 

 to the light, would probably in like manner prevent the 

 action of the developer. 



On the other hand, it is well known to photographers, 

 that the presence of a small quantity of free nitric acid 

 greatly helps the action of light on the iodides, chlorides, 

 and bromides of silver. It is not difficult to discover the 

 part which this free nitrate performs. Confining attention 

 to the case of the iodide of silver, with a slight admixture 

 of the nitrate applied to the collodion film, and exposed 

 while moist to the action of light, we must suppose that, 

 while the silver and iodine ultimates vibrate against each 

 other, a similar vibratory condition is established as between 

 the ultimate of silver, and the molecule of nitric acid 

 with its combined molecule of water. Now in the agitated 

 condition of these substances, the iodine may momentarily 

 be brought more within the attractive influence of the con- 

 stituents of the nitric acid than of the ultimate of silver, 

 and may form temporary unions with those constituents — 

 hydriodic and iodic acids, and iodide of nitrogen in small 

 quantities ; while a portion of the oxygen of the nitric acid 

 may temporarily become more intimately engaged with the 

 ultimates of silver. If a deoxidising agent be applied while 

 this state of affairs subsists, the oxygen will be easily dis- 

 engaged from the metallic silver — the iodine becoming 

 otherwise permanently occupied. In the case of the dry 

 collodion film, again, the interchange of the iodine and the 

 oxygen may become more permanent ; so that at whatever 

 distance of time the developer be applied, it has to with- 

 draw only oxygen, not iodine, from the silver. The ces- 

 sation of the action of the developer, however, after a short 

 interval, in the case of the moist film, seems to indicate 

 that, when the vibratory action excited by the light ceases, 



