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TECHNIQUE] stated that, on a couple of sheets of albuminized paper fully printed, the gold necessary to give a decided tone does not exceed half a grain.

Collodio-chloride Silver Printing Process.—In the history of the emulsion processes we stated that Gaudin attempted to use silver chloride suspended in collodion, but it was not till the year 1864 that any practical use was made of the suggestion so far as silver printing is concerned. In the autumn of that year George Wharton Simpson worked out a method which has been more or less successfully employed. The formula appended is Simpson's:—

To every 1000 parts of plain collodion 30 parts of No. 1, previously mixed with 60 parts of alcohol, are added, 60 parts of No 2 are next mixed with the collodion, and finally 30 parts of No 3. This forms an emulsion of silver chloride and also contains citric acid and silver nitrate. The defect of this emulsion is that it contains a large proportion of soluble salts, which are apt to crystallize out on drying, more particularly if it be applied to glass plates. The addition of the citric acid and the excess of silver nitrate is the key to the whole process; for, unless some body were present which on exposure to light was capable of forming a highly-coloured organic oxide of silver, no vigour would be obtained in printing. ff pure chloride be used, though an apparently strong image would be obtained, yet on fixing only a feeble trace of it would be left, and the print would be worthless. The collodio-chloride emulsion may be applied to glass, or to paper, and the printing carried on in the usual manner. The toning takes place by means of the chloride of lime or by ammonium sulphocyanide and gold, which is practically a return to the sel d'or bath The organic salt formed in this procedure does not seem so prone to be decomposed by keeping as does that formed by albumen, and the washing can be more completely carried out. There are in the market several papers which are collodio-chloride.

Gelatino-citro-chloride Emulsion.—A modified emulsion printing process was introduced by Abney in 1881, which consisted in suspending silver chloride and silver citrate in gelatin, there being no excess of silver present. The formula of producing it is as follows:— Nos. 2 and 3 are mixed together whilst warm, and No. 1 is then gently added, the gelatin solution being kept in brisk agitation ghis produces the emulsion of citrate and chloride of silver. The gelatin containing the suspended salts is heated for five minutes at boiling point, when it is allowed to cool and subsequently slightly washed, as in the gelatino-bromide emulsion. It is then ready for application to paper or glass. The prints are of a beautiful colour, and seem to be fairly permanent. hey may be readily toned by the borax or by the chloride of lime toning-bath, and are fixed with the hyposulphite solution of the strength before given. Most, if not all, of the gelatin papers now extant are made somewhat after this manner

Printing with Salts of Uranium.—The sensitiveness of the salts of uranium to might seems to have been discovered by Niepce, and was subsequently applied to photography by J. E. Burnett in England. One of the original formulae consisted of 20 parts of uranic nitrate with 600 parts of water. Paper, which is better if slightly sized previously with gelatin, is floated on this solution. When dry it is exposed beneath a negative, and a very faint image is produced, but it can be developed into a strong one by 6 to 10% solution of silver nitrate to which a trace of acetic acid has been added, or by a 2% solution of gold chloride. In both these cases the silver and gold are deposited in the metallic state. Another developer is a 2% solution of potassium ferrocyanide to which a trace of nitric acid has been added, sufficient to give a red coloration. The development takes place most readily by letting the paper float on these solutions

Self-toning Papers.—There are several self-toning papers based on the chloride emulsion process. These contain the necessary amount of gold to tone the print. The print is produced in the ordinary way and then immersed in salt and water or in some cases potassium sulphocyanide. The print is finished by immersing in weak hyposulphite of soda.

Printing with Chromates Carbon Prints.—The first mention of the use of potassium bichromate for printing purposes seems to have been made by Mungo Ponton in May 1839, when he stated that paper, if saturated with this salt and dried, and then exposed to the Sun's rays through a drawing, would produce a yellow picture on an orange ground, nothing more being required to fix it than

washing it in water, when a white picture on an orange ground was obtained. In 1840 Edmond Becquerel announced that paper sized with iodide of starch and soaked in potassium bichromate was, on drying, more sensitive than unsized paper Joseph Dixon of Massachusetts, in the following year, produced copies of bank-notes by using gum arabic with potassium bichromate spread upon a lithographic stone, and, after exposure of the sensitive surface through a bank-note, by washing away the unaltered gum and inking the stone as in ordinary lithography. The same process, with slight modifications, has been used by Simonau and Toove of Brussels, and produces excellent results. Dixon's method however, was published in the Scientific American for 1854, and consequently, as regards priority, it ranks after Fox Talbot's photoengraving process (see below), published in 1852. On the 13th of December 1855 Alphonse Poitevin took out a patent in England, in which he vaguely described a method of taking a direct carbon print by rendering gelatin insoluble through the action of light on potassium bichromate. This idea was taken up by John Pouncey of Dorchester, who perhaps was the first to pro uce veritable carbon-prints, notwithstanding that Testud de Beauregard took out a somewhat similar patent to Poitevin's at the end of 1857.

Pouncey published his process on the 1st of January 1859; but, as described by him, it was by no means in a perfect state, halftones being wanting. The cause of this was first pointed out by Abbé Laborde in 1858, whilst describing a kindred process in a note to the French Photographic Society. He says, “In the sensitive film, however thin it may be, two distinct surfaces must be recognized-an outer, and an inner which is in contact with the paper. The action of light commences on the outer surface; in the washing, therefore, the half-tones lose their hold on the paper and are washed away.” J. C. Burnett in 1858 was the first to endeavour to get rid of this defect in carbon printing In a paper to the Photographic Society of London he says, “There are two essential requisites (2) that in printing the paper should have its unprepared side (and not its prepared side, as in ordinary printing) placed in contact with the negative in the pressure-frame, as it is only by printing in this way that we can expect to be able afterwards to remove by washing the unacted-upon portions of the mixture. In a positive of this sort printed from the front or prepared side the attainment of half-tones by washing away more or less depth of the mixture, according to the depth to which it has been hardened, is prevented by the insoluble parts being on the surface and in consequence protecting the soluble part gom the action of the water used in washing; so that either nothing is removed, or by steeping very long till the inner soluble part is sufficiently softened the whole depth comes bodily away, leaving the paper white.” This method of exposing through the back of the paper was crude and unsatisfactory, and in 1860 Fargier patented a process in which, after exposure to light of the gelatin film which contained pigment, the surface was coated with collodion, and the print placed in warm water, where it separated from the paper support and could be transferred to glass. Poitevin successfully opposed this patent, for he had used this means of detaching the films in his powder-carbon process, in which ferric chloride and tartaric acid were used. Fargier at any rate gave an impetus to carbon-printing, and J. W. Swan took up the matter, and in 1864 secured a patent. One of the great features in Swan's innovations was the production of what is now known as “carbon-tissue,” made by coating paper with a mixture of gelatin, sugar and colouring matter, and rendered sensitive to light by means of potassium or ammonium bichromate. After exposure to light Swan placed the printed carbon-tissue on an india-rubber surface, to which it was made to adhere by pressure. The print was immersed in hot water, the paper backing stripped off, and the soluble gelatin containing colouring matter washed away. The picture could then be retransferred to its final support of paper. In 1869 J. R. Johnson of London took out a patent in which he claimed that carbon-tissue which had been soaked in water for a short period, by its tendency to swell further, would adhere to any waterproof surface such as glass, metal, waxed paper, &c., without any adhesive material being applied. This was a most important improvement. Johnson also applied soap to the gelatin to prevent its excessive brittleness on drying, and made its final support of gelatinized paper, rendered insoluble by chrome alum. In 1874 J. R. Sawyer patented a flexible support for developing on; this was a sized paper coated with gelatin and treated with an ammoniacal solution of shellac in borax, on which wax or resin was rubbed. The advantage of this flexible support is that the dark parts of the picture have no tendency to contract from the lighter parts, which they were apt to do when a metal plate was used, as was the case in Johnson's original process With this patent, and minor improvements made since, carbon-printing has arrived at its present state of perfection.

According to P. E. Liesegang, the carbon-tissue when prepared on a large scale consists of from 120 to 150 grains of gelatin (a soft kind), 15 grains of soap, 21 grains of sugar and from 4 to 8 grains of dry colouring matter The last-named may be of various kinds, from lamp-black pigment to soluble colours such as alizarin The gelatin, sugar and soap are put in water and allowed to stand for an hour, and then melted, the liquid afterwards receiving the 