IN chapter IV (The Light Sensitive Materials Used In Photography) we saw that the chemical process of development consists of the removal of the bromine from the silver bromide in the emulsion so as to leave the grains of silver behind.

There are many chemicals which will remove bromine from silver bromide in this way, but in order to act as a developer, it is necessary that a chemical should be chosen which has the power of turning the exposed silver bromide into metallic silver, but which will not act on unexposed silver bromide, since, if the developer acted on the unexposed, as well as on the exposed grains, we should not get an image at all, but the whole film would go dark when put in the developer, just as if it had all been fogged by exposure to light. Only a very limited number of chemicals have this power of distinguishing between exposed and unexposed grains of silver bromide and, consequently, there are only a few substances which are suitable for use as developers.

The chief of these developing substances are pyrogallol, or "pyro" as the photographer calls it, hydroquinone and elon, all of which are chemically related to aniline, which is used as the base of coal tar dyes. Hydroquinone and elon, indeed, are made by the same methods as those used for making dyes, but pyro is made by distilling gallic acid, which is produced by fermenting gall nuts, so that, although pyro is really a cousin of hydroquinone, it is made quite differently, from a vegetable product, while hydroquinone itself is made from aniline.

Now, if we take a solution of one of these chemicals, let us say pyro, and put an exposed film into it, we shall get no development at all; the developing agent by itself having no power to develop. In order to make it develop we must add a little alkali to the solution. Any kind of alkali will make it develop, but the most convenient one to use is carbonate of soda which, in its crude form, is called sal-soda and is used to make water alkaline for washing. If, then, we take a solution of pyro and add some sodium carbonate to it it will develop our exposed films; but a solution containing only pyro, carbonate and water will not keep and, if we leave it in the air, it will very soon darken and lose its developing power.

In order to make it keep, there is added to the developer some sulphite of soda because the developer is spoiled by taking up oxygen, and sulphite is so greedy for oxygen that it will take it away from the oxidized pyro or take it in preference to the pyro, and thus protects the pyro from the oxidizing action of the air and enables it to keep its developing power, although the sulphite itself has no developing power at all.

The essential constituents of a developer therefore are: The developing agent - pyro or hydroquinone or elon or Kodelon which is a relative of elon - the alkali, which is generally carbonate of soda, and the preservative, which is sulphite of soda. Very often a developer which contains only these constituents will prove difficult to handle. It will tend to give fog, that is, to develop unexposed silver bromide as well as exposed silver bromide, and so, in order to regulate it, there is put in a little potassium bromide to act as a restrainer.

The various developing agents behave somewhat differently. Suppose, for instance, that we make up two developers, one with hydroquinone and the other with elon, and start to develop a film in each at the same time. In the elon developer the image will appear very quickly on the film and will appear all over the film at the same time, the less exposed portions which, of course, were the shadows in the picture, appearing at the same time as the highlights. On the other hand, with the hydroquinone the image will appear more slowly, and the most exposed portions, or the highlights, will appear first, so that by the time the shadows have appeared on the surface of the film the highlights will have acquired considerable density. If development is stopped as soon as the whole image is out, then the negative developed in elon will be very thin and gray all over, while that developed in hydroquinone will have a good deal of density in the highlights. Thus, of these two developers we may say that elon gives detail first and then slowly builds up density, while with hydroquinone the detail comes only after considerable density has been acquired. It is for this reason that these two developing agents are used in combination; the hydroquinone gives the density and the elon the detail, and together they make a well balanced developer.

These differences in the behavior of developing agents are due to a property of the developer which can be explained very easily by an analogy. Suppose that we had two automobiles of the same kind, one of 20 horse power and the other of 100 horse power. What would be the difference between them? Naturally, the high horse power automobile would be able to go faster than the other; but in a city, at any rate, either of them would be able to go as fast as was safe, and no one would wish to use the higher horse power for increased speed; but the advantage of the high horse power would be found whenever the automobiles were used against adverse circumstances, as, for instance, against high winds, in snow or in climbing hills, when the high-power machine would be able to keep up its speed against the difficulties, and the lower power machine would be slowed and might even be unable to get ahead. The difficulties which affect development in a manner corresponding to the effect of hills or winds for an automobile are cold and bromide. The addition of bromide has the same effect on a developer that a hill has on an automobile - it slows it down; but bromide has far more effect on a low power developer like hydroquinone than it has on a high-power developer like elon; the effect of bromide on elon is very small, while on hydroquinone it is very great. In the same way, hydroquinone develops very slowly when it is cold, while elon is not nearly so much affected by temperature.