Although metol can develop in a non-alkaline solution of sulphite, it is usually rendered alkaline by the alkali carbonates. Its energy becomes excessive with a caustic alkali unless very small quantities are used, and there is a danger of chemical fog unless a fair amount of bromide is used, which precaution is often unnecessary with a carbonate, provided that the developer does not contain too much metol. Developing solutions containing metol and alkali carbonates keep well. They are well suited for developing negatives known to be under-exposed. Contrary to a widely-held opinion, it is quite possible to obtain very contrasting images, provided the plate is left sufficiently long in the bath. The contrast is less than with hydroquinone alone, but with the latter the effect is obtained partly at the expense of details in the shadows. 2 As a rule, metol is mixed with hydroquinone (§ 349), the developing solution thus obtained being, from various points of view, distinctly superior to those in which either one or other of these developers is used alone. It keeps very well, does not stain either the fingers or the gelatine, gives as much detail as metol used alone, and nearly as much contrast as hydro quinone.
The skin troubles (similar to eczema) attri buted to metol are exc usively due to an im purity (NN—dimethyl paraphenylene diamine) and do not appear to have been experienced with metol of French make.
Glycin. Very brilliant, colourless, crystalline scales, somewhat resembling mica, and melting and decomposing at about 200° C. (392° F.). Glycin is almost insoluble in cold water and in alcohol, insoluble in ether, soluble in dilute solutions of mineral acids (but not in acetic acid), soluble also in solutions of sulphites and alkali carbonates (with effervescence if glycin be present in quantity) and in solutions of caustic alkalis.
The chief characteristic of glycin is that it is practically inoxidizible by air either in the dry condition or in carbonated solutions, even when very dilute. It is therefore admirably suited for prolonged time development in very dilute solutions, especially as only a very small quantity of sodium sulphite need be added. This avoids the risk of dichroic fog, common with very dilute developing solutions of easily oxidized developers with which a proportionately larger amount of sulphite has to be used. For the same reasons it is very suitable for the mechanical development of cinematograph film on continuous machines, or on revolving drums which are partially immersed.
In preparing developing solutions with glycin, potassium carbonate is usually preferred to sodium carbonate, since it is then possible to make up more concentrated stock solutions.
Eikonogen. At the time of its introduction eikonogen enjoyed great popularity. It lost much of its popularity on the introduction of metol, and is now used only in Germany, the only country where it is manufactured. The
same inay be said of a developer of almost the same constitution, Diogen (acid arninonaphthol disulphonate of sodium).
361. Amidol. Diaminophend. Though be longing to the aminophenol class, this developer owes to its two amine functions very special properties which are possessed also by a closely - related substance, Diaminoresorcine, formerly somewhat popular.
Diaminophenol hydrochloride occurs in crys talline needles, which are colourless or light grey (incipient oxidation, difficult to avoid in manu facture) and decompose without melting when heated. This salt is very soluble in water and almost insoluble in alcohol and ether. It dis solves freely in solutions of sodium sulphite, which are thereby partly transformed into bisulphite. This solution oxidizes fairly rapidly in air and must therefore be prepared shortly before use. The neutralization, and particularly the alkalinization, of this solution considerably hasten its oxidation, which shows itself by a very deep colour (deep blue for the carbonates and wine red for the caustic alkalis). On the other hand, an addition of bisulphite or of a weak acid (boric, lactic, glycollic, etc.) confers fairly good keeping qualities, the mixture being capable of being kept for several days in au open container without any appreciable deterioration. 1 Diaminophenol hydrochloride, as we have already seen (§ 348), will act as a feeble developer in plain water solution. It develops fully when made up with sulphite, the speed being the greater as the solution is less acid. 2 As long as the mixture contains neutral sulphite, develop ment proceeds, as with all usual developers, downwards from the free surface of the emulsion. On the other hand, when the neutral sulphite has been entirely coverted into bisulphite (by the hydrochloric acid of the developer itself, or by acids separately added), development is very slow, begins in the deepest layers of the emulsion, and only slowly reaches the surface, the image being visible first on the non-emulsion side (G. Balagny, 1912). This depth development takes place only so long as the developing bath has a distinct smell of sulphur dioxide (smell of burning sulphur). It is based on the fact that the reduction of the silver bromide becomes possible only after the gelatine of the upper layers of the film (by combining with the sulphurous anhydride) allows the liquid filtering through to the deep layers to free itself from the excess of acid which prevents Developing solutions of amidol produce images of a perfectly neutral grey-black. Their continuous use, at least in non-acidified solu tions, tends to stain the fingers and nails a blackish brown.