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182 THE ADVANCE OF PHOTOGRAPHY
these with pyrogallol, although, if desired, they may be used with hydro quinone, which also requires an alkaline solution.
With reference to the carbonate of soda, which is the alkali often used, it should be remembered that, as given in ordinary formula?, it is the crystalline form which is generally understood, although some formula? give particulars for the dry powder as well. The anhydrous form is much more powerful in action, since the loss in weight which takes place when the crystals change to a powder is due to the loss of the water of crystallization.
It is always advisable to use the developer at normal strength for all properly exposed plates, unless it is desired to produce a soft negative. When this is the case the developer should be somewhat diluted. The diluted developer will, of course, be slower in its action than the normal.
Hydroquinone is another well-known developer, and is especially useful when greater density is required. Hence this developer is often used when making transparencies and lantern slides. It should be remembered when using hydroquinone that negatives developed in that substance lose more than usual in density when undergoing fixing.
This developer is somewhat slow in its action, especially at low temperatures, and it is often found better to use a combination consisting of hydroquinone and metol. The latter is an extremely active developer, and the compound developer is a very useful one where delicate soft negatives are required.
It is peculiar that metol brings out the details first and afterwards increases the density of the negative ; such a developer is very handy where the object aimed at is to obtain excellent detail rather than strong contrasts.
These two developers belong to the non-staining class, and can therefore be used for the development of bromide papers, etc., as well as plates.