Now, if we consider what happens when a plate is developed, we shall at once see that at first we shall have some silver halide grains, which are so affected by light as to be readily reduced by the developer, and some which are not affected. As development proceeds we shall have (a) some grains already reduced to metallic silver, (b) others not yet completely reduced, and (c) others which are not light-affected and there fore not attackable by the developer. Natur ally, at first the progress of development will be rapid, as the whole of the light-affected grains will be capable of development, but as it pro ceeds there will be fewer and fewer of the (b) grains, so that development gradually gets slower and slower, and we may express this by saying that: the rate of increase of density = constant (maximum density attainable density obtained).
Here the constant or velocity constant is usually termed K.
If the temperature of the developer be raised then the velocity constant, or K, increases, and this is termed the temperature coefficient, which is generally defined as the ratio for correct development at io° C. (5o° P.) difference of temperature.
The efficiency of a developer E is the velocity of development compared with ferrous oxalate at C., divided by R, which is the reducing power, or the number of grain-molecules of AgBr reduced by one grain-molecule of the developing agent. The energy P is the con centration of bromide producing the same retardation of development as with ferrous oxalate for o•or n potassium bromide. For a complete mathematical treatment of the subject, the reader is referred to " Theory of the Photographic Process," by Sheppard and Mees.
Before leaving this subject it would be as well to consider the question of the chemical constitu tion of the actual developers or reducing agents.
Of late years the number of developers has been largely increased, and a glance at their true chemical names will at once prove that they are highly complex organic substances belonging to the benzole or naphthalene series.
Benzene or benzole has the formula but in 1865 Kekule, from a long series of experiments, came to the conclusion that the six atoms of carbon in benzene form a closed-chain or nucleus, and that the molecule of benzene is symmetrical, and that each carbon atom is directly united with one, and only one, atom of hydrogen. The graphic formula usually adopted is as follows :— Now, in the above formula it is obvious that if two of the six hydrogen atoms were replaced by two other atoms or groups they might be arranged in one of five different ways, as shown in the accompanying diagrams, in which, for the sake of clearness, the C atoms are omitted and the added group or atom expressed by x.
Then, if one x group occupies any given position, that numbered 1, for instance, the other may occupy 2, 3, 4, 5, or 6. But these five formulae only represent three isomeric compounds, that is, compounds of the same composition and not five, because it is obvious that IV. and V. are practically identical with I. and II., which may be at once seen by writing them on thin paper and looking at them first in the normal way and then through the paper. In order to distinguish
these three compounds, they are said to be in the ortho, meta, and para positions :— Now, obviously in the ortho position the two z groups are joined to carbon atoms which are directly united or are next to one another, so, that we could actually represent an ortho compound as halide atoms Cl or Br, then the developing power is increased, as in the case of Exactly in the same way, the meta compounds could be represented by placing x x at 1-3, 2-4, 5-1 and the para at 2 — 5 , or 3-6 positions.
Now, it has been proved by Lumiere and Seyewetz and by Andresen that a developer must have two hydroxyl OH groups or two amido groups joined to the benzole ring, and that these groups must be either in the ortho or para positions to be developers. The latter compounds are the stronger developers, the ortho compounds the weaker, whilst the meta compounds are not developers at all. If one of the hydrogen atoms of a hydroxyl group be replaced by an alkyl radicle—that is, an alcohol radicle, such as ethyl by another radicle, then the developing power is destroyed, as, for instance, in phenetroin if, on the other hand, one of the hydrogen atoms of an amido group be replaced by an alkyl radicle, then the developing power is increased, as in the case of Besides the OH and groups, the hydrogen atoms of the ben2ole ring may be replaced by other radicles, such as carboxyl C0011, and the sulpho group but these reduce the developing power, and in this case the position of the added or substituted radicle plays an important part, as in the instance of If three hydrogen atoms in the benzole ring are replaced by three or OH groups, then the developing power is increased, and here, as in the case of the di-substitution products, the ortho-, meta-, or para position plays an important part, thus because in the former there are only two OH groups in the ortho position, whilst in the latter there are three OH groups all in the ortho position. Again, if one OH and one group be added to the ring, then we have a still more energetic developer, as in the case of whilst the addition of another group creases the action still further, as in the case of which is at once an ortho- and a para-amido phenol, and diamidoresorcin the former being but a very weak developer, whilst the latter is a vigorous one.
If a hydrogen atom be replaced by one of the a double para-amidophenol, in which the OH and groups, as shown by the lines, are in two para positions; this is still more energetic.
There are two developing agents which belong to the naphthalene H8 series, and naphthalene may be considered as two benzene rings joined together, and which at the points of junction have lost their hydrogen atoms, so that we may write naphthalene graphically as Here, too, the hydrogen atoms may be replaced by other atoms or groups, and we have It will be easily seen that the Greek letters and the numbers refer to the positions of linking of the substituted atoms. In the above sketch the developing bases only have been considered, for many of the actual developers are salts, such as chlorides, sulphates, oxalates, etc., of these bases.