Eastman Portrait Film Negative, Artura Print By Chas. A. Townsend Belfast, Maine.

Eastman Portrait Film Negative, Artura Print By Chas. A. Townsend Belfast, Maine.

Eastman Portrait Film Negative, Artura Print By Chas. A. Townsend Belfast, Maine.

Eastman Portrait Film Negative, Artura Print By Chas. A. Townsend Belfast, Maine.

Different reducing agents behave differently in development. We cannot use Elon in the place of hydroquinone and get the same effect. An image developed with Elon comes up very quickly all over the plate and gains density slowly, while the hydroquinone image comes up very slowly but gains density steadily and rapidly. A very little change in the temperature affects hydroquinone a good deal and affects Elon very little, and in the same way a small amount of sodium or potassium bromide affects hydroquinone and does not effect Elon nearly so much. These differences in the developing agents depend upon the chemical nature of the substances themselves, and the particular property to which these differences are due is called the "reduction potential" of the developer.

The reduction potential does not by itself determine the speed with which the developer develops the image, because this depends chiefly upon the rate at which the developer diffuses into the film, and on the amount of developing agent and other substances in the developer. A high reduction potential enables a developer to continue to develop more nearly at a normal rate under adverse circumstances, such as low temperature or the presence of bromide. The reduction potential of a developer, in fact, may be compared to the horse power of an automobile which for other reasons than the power of its engine is limited in speed. If we have two automobiles and they are confined to a maximum speed of twenty miles an hour, then on level roads the one with the more powerful engine will be no faster than that with a weaker engine, but in a high wind or on a more hilly road the more powerful engine will allow the automobile to keep its speed, while the machine with the weaker engine will be forced to go more slowly. We could, indeed, measure the horse power of an automobile by the maximum grade which it could climb at a uniform speed of 20 miles an hour.

In development, the analogy to the hill is the addition of bromide to the developer, since the addition of bromide greatly delays development, and it is found that the higher the reduction potential of a developer the more bromide is required to produce a given effect. If we measure the developing agents in this way, we shall find that hydroquinone has the lowest reduction potential, then pyro, then Kodelon, and finally Elon has the highest. Hydroquinone has so low a potential that it is rarely used alone but is generally used with Elon. Kodelon can be substituted for Elon but more Kodelon has to be used in order to produce a developer of the same strength. Developers with a high reduction potential such as Elon, and to a less extent Kodelon, make the image flash up all over at once because they start development very quickly even in the lesser exposed portions of the emulsion, while developers of low reduction potential, like pyro and especially hydroquinone, bring up the high-lights of the image first and the shadows do not fully appear until the highlights are somewhat developed.

Eastman Commercial Ortho Film Negative, Artura Print By Chas. A. Townsend Belfast, Maine.

Eastman Commercial Ortho Film Negative, Artura Print By Chas. A. Townsend Belfast, Maine.

Developing agents cannot develop at all when used by themselves. With the single exception of Acrol, developing agents in order to do their work must be in an alkaline solution, and the energy depends upon the amount of alkali present. The developers of higher reduction potential, which bring up the image very quickly, require less alkali than those of lower reduction potential. For instance, hydroquinone is often used with caustic alkalis, while the other developing agents require only the weaker carbonated alkali.

The amount of alkali governs the energy of a developer, and if too much alkali is present, the developer will tend to produce chemical fog, while if too little alkali is present, it will be slow in its action. Alkalis also soften the gelatine of the emulsion, and consequently too alkaline a developer will produce over-swelling and will give trouble with frilling or blisters in warm weather. This action of the alkali on the gelatine has nothing to do with its developing properties but is merely an unfortunate fact.

The alkalis used in development are of two kinds: the caustic alkalis and the carbonated alkalis.

Caustic alkalis are produced when the metal itself reacts with water, the metals from which the alkalis generally used are derived being potassium and sodium. These metals are so easily oxidized that they have to be preserved from all contact with air or water by immersion in light oil or gasoline.

If we take a small piece of sodium and place it on the surface of water in a dish, it will react with the water with great violence, melting with the heat produced and buzzing about the surface, while if we restrict its movement, the development of heat will be so great that the hydrogen produced will burst into flame. In the case of potassium, the reaction is even more violent than with sodium and is always accompanied by flame. The reaction may be represented by the equation Na + H2O = NaOH + H Sodium Water Caustic Soda Hydrogen the sodium combining with the water to form caustic soda and liberating hydrogen, which comes off as gas, and, as has already been stated, catches fire and burns in the air. This is, of course, not the method by which the alkalis are actually produced. As a matter of fact, the metals are produced by electroplating the metal out from the melted alkali.

Eastman Commercial Ortho Film Negative, Artura Print By Chas. A. Townsend Belfast, Maine.

Eastman Commercial Ortho Film Negative, Artura Print By Chas. A. Townsend Belfast, Maine.

Caustic Soda is made either by the passage of anelectric current through a solution of common salt, when the soda separates at one electrode and chlorine gas is liberated at the other, or from sodium carbonate, which is causticized by means of lime. Lime is calcium oxide and is prepared by heating limestone, which is calcium carbonate, the carbon dioxide being driven off from the limestone by the heat. When the lime is added to sodium carbonate, the lime removes the carbon dioxide from the carbonate, and leaves the sodium hydrate in the solution, which is then evaporated to get the solid substance. At present, caustic soda is easily obtained in a very pure state, and there is usually no difficulty in getting good caustic soda for photographic work. It must be protected from the air, since it easily absorbs moisture and carbon dioxide. As its name indicates, it is very caustic and attacks the skin, clothing, etc.

Caustic Potash is very similar to caustic soda and is prepared in the same way. Fifty-six parts of caustic potash are chemically equivalent to forty parts of caustic soda. (To be continued.)