Fig. 71 represents an automatic apparatus devised by Gorceix for washing photographic negatives. It consists of two triangular prismatic troughs, placed back to back, oscillating on a rod passing through them a little above their centre of gravity, and supported by two uprights. The oscillation causes the partition common to the two troughs to take two positions, one to the right and the other to the left, that make an angle of about 15° with the vertical. The uprights are fixed to the sides of a box provided with a waste tube, and are connected at their upper part by a cross-piece, that carries a tube through which water continuously enters the troughs. The water, on entering the compartment below, raises the centre of gravity laterally, and when the latter gets above the axis, the troughs tilt. A part of the water is thus emptied into the box, while the rest, being held back by a narrow rim, covers the plate and enters the gelatine.

Fig 71.

Automatic Apparatus For Washing Negatives 10052

As a consequence of the rocking motion, the other trough is brought under the stream of water and becomes full in its turn, and the two troughs then tilt in the opposite direction. These motions continue as long as the water flows, and their frequency depends upon the discharge, which may be regulated at will by means of a cock.

It will be seen that when once operations have begun, the washing proceeds very regularly. The negative soaks for a certain length of time in the water, and gives up to it a portion of the salts that it contains, and then a larger bulk of water dilutes the solution and is in part ejected, so that the strength of the liquid lowers so rapidly that there are soon only inappreciable traces of solids in the water, and the washing is then finished. When it is a question of ridding paper proofs of soda hyposulphite, the troughs are covered with wire cloth with wide meshes, which holds the proofs without interfering with the circulation of the water.

The apparatus is made of zinc covered with bitumen, which is afterward exposed to the sun. Making Gold and Silver Salts. - The salts of gold and silver used in photography are expensive to buy, and their manufacture is not difficult, even if undertaken by a non-chemical person, if he follow closely the directions about to be given.

There are two chlorides of gold - the protochloride and the terchloride. The latter salt is the one used in photography. The chemical formula of the protochloride is AuCl, and that for the terchloride is AuCl3 (that means that in the terchloride there is three times as much chlorine united with the same amount of gold as that in the protochloride). The terchloride is the most important compound of the metal. It is always produced when gold is dissolved in a mixture of nitric and hydrochloric acids. Gold is not dissolved by any simple acid, nor by any other liquid than this nitro-hydrochloric acid. The deep yellow solution produced by dissolving gold in this compound acid yields by evaporation yellow crystals of the double chloride of gold and hydrogen. When this is cautiously heated, hydrochloric acid is expelled, and the residue on cooling solidifies to a red crystalline mass of terchloride of gold, which is very deliquescent (that is, it imbibes moisture), and is soluble in water, alcohol, and ether. The terchloride combines with a number of metallic chlorides, forming a series of double salts. These compounds are mostly yellow when in crystals, and red when deprived of water.

A mixture of terchloride of gold, with excess of bicarbonate of potash or soda, is used for gilding small ornamental articles of copper. These are cleaned by dilute nitric acid, and then boiled in the mixture for some time, by which means they acquire a thin but perfect coating of reduced gold.

The yellow needle-shaped salt usually obtained by evaporating the acid solution to the crystallising point is an acid chloride (formula HClAuCl3). As it is not absolutely necessary that perfect purity of the chloride is essential for photographic purposes, the salt can be made from any scraps of gold ornament of 18 to 22 carats fine. It would not be worth while to employ a lower standard, since the larger amount of alloy in 15, 12, and 9 carat gold would too much interfere with the production of good terchloride; and, moreover, the small amount of gold present would not be of sufficient quantity to repay the trouble of manipulation. If small quantities of the salt be required, a book or two of gold leaf can be employed for its production. If larger quantities, a gold coin may be used.

The atomic weight of gold is 197, and that of chlorine 35.5 - that is 197 parts of gold unite with 35.5 parts of chlorine to form the protochloride (AuCl), and in the terchloride 197 parts. of gold unite with 106.5 (i.e. 35 • 5 x 3) of chlorine to form the auric chloride (AuCl3); consequently, the combining weight of the latter salt is 303.5. The Australian gold coins are alloyed with silver, and therefore are preferable to those of the English mint, inasmuch as the silver is left in the shape of undissolved chloride, which can afterward be filtered out, whereas copper is much more difficult to get rid of.