1Thus, suppose a certain change requires 5 minutes for a completion at o°C.

At 100° it will proceed 2 100/10=1024 times as fast and be complete in about 3/10 seconds.

2The weight of matter involved and the exothermic surplus are too small to cause any noticeable rise of temperature of the developing bath.

Fixing

When development is complete, the unchanged AgX must be removed, for two reasons: It is opaque. It darkens on exposure to light, becoming still more opaque. The AgX goes into solution when the plate is immersed in a solution of sodium thiosulphate - known as "hypo" - (the fixing bath).

The action of Na2S203 on AgX is not exactly known. We quote, however, some English text-books.

Na2S2O3 + 2 AgX = Ag2S203 + 2NaX.

Silver thiosulphate is sparingly soluble in water, but readily soluble in Na2S203 solution.

Na2S2O3 + Ag2S203 = Na2S203 . Ag2S203.

These equations are close enough for the photographer's purpose, for they show why a second immersion in "hypo" diminishes the time necessary for washing.

It is of the utmost importance that all the S203 radicle be removed from the film. Thiosulphates are slowly attacked by carbon dioxide from the air, with liberation of finely divided sulphur, and also of sulphur dioxide.

Na2S203 + C02 + H20 = Na2CO3 + S + S02.

This sulphur will combine with the silver and lessen its opacity. Any sulphur dioxide remaining in the substance of the film will also go through a number of chemical changes, none of which will do any good, either to the image or to the gelatine.

Action Of The Sulphite

The chief function of sodium sulphite in solutions of organic developers, is to prevent stains on the film. Stains are the result of secondary reactions undergone by the compounds formed when aromatic developers are attacked by hydroxyl; these bye-products tend to oxidise and then undergo condensation with the consequent formation of dyes. The ordinary action of sodium sulphite keeps these dyes in the leuco condition - in which state they are colourless - thereby preventing stains. It has generally been thought that the sole action of sodium sulphite was to prevent weakening of the developer, owing to the superior avidity with which it will take up any oxygen going into solution from the air. And it was pointed out that alkaline pyrogallol, which absorbs oxygen very rapidly, requires the largest quantity of sulphite, whereas ferrous oxalate, which absorbs little oxygen, requires none. But though it thus enables a developer to "keep" better, this fact is of quite minor importance in comparison with its action in preventing stains.

Printing In Silver

With the ordinary daylight printing paper, the basis of which is silver chloride, exposure is very long, and the image (probably?) consists of metallic silver. From the reddish colour of an untoned print one would suggest that the silver was in the form of one or more of its allotropic modifications. Toning consists in the partial replacement of the silver image by a gold one, according to the equation:

AuCl + Ag = AgCl + Au.

The change of colour is explained by the bluish tinge of very finely divided gold. The gold solution must be very dilute, and in practice some substance, usually an organic sodium salt, is added to prevent the destruction of the silver (or subchloride) image.

The theory of the fixation of prints is the same as that of negatives, the removal of the unchanged AgX. In fixing these prints it is advisable to use a slightly alkaline solution of thiosulphate. This body is unstable in the presence of acids, sulphur being liberated, which will combine with the silver and spoil the print.

Besides these printing-out papers there are also on the market a wide variety of "bromide" and "gaslight" papers. These are exposed and developed in the same way as negatives. The image is ordinary silver in one or more of its allotropic modifications : whence the multiplicity of shades obtainable.

Printing In Platinum

We breathe a sigh of relief in turning to the platinotype process. Here we are on firmer ground. Two very simple chemical changes underlie this process. The first is the reduction of ferric salts to the ferrous state by the action of light; and the second, the re-oxidation of the ferrous salt at the expense of the acid radicle in combination with platinum, the platinum thus precipitated in the metallic form building up the image. Details vary with different makes of paper; we need only describe the simplest process. A paper containing a mixture of ferric oxalate and potassium chloroplatinate1 is exposed to light behind a negative in a printing frame. Local reduction of the ferric oxalate takes place in accordance with the equation :

(COO)6Fe2 = 2(COO)2Fe + 2C02.

No further change can now take place in the dark so long as the paper is dry, for the ferrous oxalate and the platinum salt cannot react in the solid state. They will do so, however, the instant they enter into solution.

Such papers, therefore, are developed by immersion in water.2

6(COO)2Fe+ 3K2PtCl4 = 3Pt + 2(COO)6Fe2 + 2FeCl3 + 6KC1.

This change, occurring at the moment of solution, the platinum is deposited in the pores of the paper. Fixation takes place by the solution of all the salts left in the paper. But a little dilute hydrochloric acid must be used in the washing water, to prevent the hydrolysis of ferric salts, especially chloride, which occurs in neutral solution.

FeCl3 + 3H20 = Fe(OH)3 + 3HCl..

1 K2PtCl4 is readily soluble. The simpler body PtCl2 is unsuitable, because sparingly soluble in water.