To conclude, we can only assert of the latent image that it consists of some unknown substance, which is acted on by the developer at a different rate to the unchanged silver halide, and on that account acts as a catalyst. We cannot say whether it is acted on more, or less, rapidly, but shall find this point merely enhances the pleasure of sketching the theory of development.
Action of Light on the Plate.—Before proceeding to discuss development, we should like to clear the air a little as to the chemical effect of light. The actinic power of light is so often spoken of as if it were something not-to-have-been expected, some new manifestation of energy. And this while two facts concerning light are common knowledge. We have but to mention the name of Clerk Maxwell and the reader at once remembers that light is an electrical phe nomenon.
One expects some chemical change to take place when an electrical disturbance is propagated through an ionised solution.' The other well-known fact is that heat is developed when light is absorbed by a medium. This local heating will have two effects. It will first increase the number of free ions, thereby aiding electrolysis. Secondly, it will tend to vaporise any bromine or iodine that may be liberated, and by thus removing it from the sphere of chemical action, render recombination with the silver impossible.
Development of the has been shown that it is necessary to formulate two theories of development, since we do not know whether the unchanged or the changed silver compounds on the plate are attacked the more rapidly by the developer. The general law which has been found to apply in all such cases is that the less active substance is unaffected, whilst the more active one is attacked at an increased rate, which increase is directly proportioned to the difference in chemical activity between the two substances.
On the supposition that the photo salt is attacked less rapidly than the unchanged silver halide, the simplest explanation of development lies along the lines laid down by Desalme' Desalme's Theory of Development is electrolytic. It is found that whenever two substances that differ either chemically or physically, are placed in electrical contact, a difference of potential is set up between them. (In the case of an exposed plate this potential difference between the photo-salt' and the unchanged AgX 2 will be increased by the action of the electrolyte.) If this E. M. F. be high enough electrolysis will commence. But, immediately, owing to the accumulation of anions and cations in opposite regions, an equal back E. M. F. will spring into existence, and cause polarisation. Electrolysis ceases. To re-establish it, a depolariser is needed. It will be enough to remove the bromidion, by causing it to enter into chemical combination.
It has generally been assumed that the bromidion combines with the developer. This is true in the case, f ferrous oxalate : 3 Re ea_ey _3 A 3(CO t. e + 3 AgX = + (CO 3 rig but untrue for the usual organic was thought that the halogen attacks the organic developer, forming a halogen substitution compound, with liberation of HX, which was neutralised by the alkali, whereby the action of HX on the silver of the image was prevented. But if this were so, then we should find that bodies which react most readily with the halogens (such as the monohydrophenols) would make very good developers ; but such is not the case, these compounds do not develop. Furthermore, if the bromidion combined with the developer, we should expect unsaturated compounds, such as cinnamic acid