they insert the features of the ground, suitably delineated according to the scale.

A very early application of this process has lately been re-introduced in Germany by Herr Romain Talbot, of Berlin, under the title of the Lichtpaus process, with the object of enabling engineers and others to readily prepare a few copies of their plans without the necessity of using a camera and other expensive appliances. In this method a print on a sensitive chlorised paper, prepared with nitrate of silver and an organic acid, so that it may be kept for some time in stock ready for use, is first taken by exposing it to light under the original drawing itself, which to secure the best results should be drawn in very black ink on thin paper or vellum cloth. This print, on which the lines are clear and the ground opaque, is simply fixed in a solution of hyposulphite of soda and then thoroughly washed and dried. A second copy is now made from this negative print in exactly the same way, and as, this time, the lines darken under the clear parts of the negative and the ground remains clear, we obtain a perfect transcript of the original. This process is said to be largely used in Germany for copying maps and engineering plans. It is no doubt useful in many cases where it is undesirable or impossible to make more extensive photographic arrangements, but besides being limited to the reproduction of copies on the same. scale as the original, it labours under the disadvantages of expense and want of permanency common to all the silver printing processes.

In the processes just noticed the exposure to light is usually from 15 to 30 minutes, but in dull weather, or with certain negatives, it may be much longer; it is obvious, therefore, that even under the most favourable conditions comparatively few prints can be produced from a single negative. in a day. In order to shorten the exposure and permit prints to be produced with much greater rapidity than with the ordinary process, a method has been introduced by Major Libois of the Belgian army, by which, instead of the image being produced at once in its full strength by the action of light, the latter is only allowed to act for a few seconds, and the full effect is produced by treating the print with a developing agent composed principally of gallic acid, which at once reduces the silver in the parts acted on by the light, and thus produces a visible image in place of the almost invisible one formed by the action of the light alone. This process was extensively used in the Depôt de la Guerre, Paris, some years ago, and large numbers of maps were turned out by it, I was told, almost as quickly as they could have been printed in the press, and it had the further advantage that facsimile copies could be made of maps from which good results could not have been obtained by photolithography. The same process was used at the Depôt de la Guerre in Brussels, but not on so large a scale. I have also used it with success in India, and it may be recommended in

cases where silver printing is required for maps &c. It is more economical than the ordinary process, and much more rapid in working, the exposure being counted by seconds instead of by minutes. The operations are briefly as follows:

Thin photographic paper is salted by floating on a solution containing 2 per cent each of chloride of ammonium and citrate of soda in water slightly acidified with citric acid. The paper is dried and may be kept for use. It is sensitised by floating in the dark on a bath containing 5 per cent of nitrate of silver acidified with a little citric acid. The exposure to light is conducted in the same way as in the ordinary process, but is exceedingly short—a few seconds to one minute being generally sufficient. When taken from the printing frame only a very feeble image is visible, it is therefore developed in a mixture of a solution of gallic acid (1 to 3800 water) with a solution of acetate of lead (1 to 200 water), to which a little acetic acid is added. The detail gradually strengthens, and in about a quarter of an hour the prints are fully developed and of a good black colour. After being washed they are fixed in a bath of hyposulphite of soda at 30 per cent., then well washed and dried.

*

The foregoing are the principal methods of silver printing suitable for cartographic purposes, but, however convenient and useful they may be for special objects where photolithography is not applicable, they cannot be considered adapted for purposes of publication, and their want of permanency is an insuperable defect.

Printing with Salts of Iron.-From time to time attention has been drawn to the possibility of replacing silver-printing by processes depending on the use of the salts of iron and other cheap materials, but, though certainly useful in some respects, they have never been brought into extensive practical use.

One of the best known of these processes is the 'cyanotype', invented by Sir John Herschel, and lately re-introduced by Messrs. Marion & Co., of Paris, who prepare and supply the ferro-prussiate paper ready for use. Good even-textured paper is brushed over with a mixture containing nearly equal proportions of 10 or 12 per cent solutions of ammonio-citrate. of iron and the ferrideyanide of potassium, dried and exposed to light under a print or drawing placed with the printed side uppermost. The resulting faint photographic image is developed and fixed by a mere washing in plain water, yielding a print in white or light blue lines on a dark blue ground.

This process is rapid, simple and cheap. The camera is dispensed with, and the only photographic apparatus required is a printing frame and * Maës and Hannot's Traité de Topographie et de Reproduction des Cartes au moyen de la Photographic, p. 295.

one or two dishes or trays. The sensitive paper is easily prepared and can be kept indefinitely in the dark until required for use. There is no messing with chemicals after the preparation of the paper, pure water only being required to develope and fix the prints. The exposure to the light is very short, two or three minutes in the sun being ample to make a clear legible copy from a line negative or from a drawing on tracing cloth. The chemi-t cals employed are both very inexpensive.

The objections to the process are two: first the difficulty of obtaining clear whites; this, however, is of no consequence so long as the details are clearly legible; and secondly, the colour of the prints-white on a dark blue ground. Although this does not interfere with the practical use of the process for special work, it completely prevents it from being employed as a means of multiplying copies of maps or plans on a large scale. Another defect is, the want of sharpness arising from the necessity for placing the reverse side of the original in contact with the sensitive paper in order to get an unreversed print. These objections may be partly obviated by printing from a negative on paper or glass, in which case the lines will be dark blue on a light blue or white ground, but then cameras and other expensive photographic apparatus will be required to produce the negative.

M. H. Pellet has recently recommended a process of this kind whereby prints are obtained in dark lines on a clear ground. Paper is sensitised in a mixture of—

dried and exposed as usual under a drawing. The print is developed in a bath of yellow prussiate of potash at 15 or 18 per cent, well washed and fixed with dilute muriatic acid, then finally washed and dried.

The blue prints thus produced can also be used as the basis of drawings for photozincography.

Another process, which, though not quite so simple as the above, has the advantage of giving a print in black on a white ground, forms one of the numerous important photographic methods for which we are indebted to the illustrious Poitevin.

Paper is coated in the dark with a solution of perchloride of iron and tartaric acid in water; when dry, it is exposed under a tracing on cloth or paper, or a reversed positive on glass, and as soon as the parts exposed to the light have become thoroughly bleached the print is removed and developed in a bath of gallic acid. The parts protected from the light turn to an inky black, while the exposed and bleached parts remain white or only take a slight tint. The print is then thoroughly washed and dried. The whole.

operations of printing, developing and washing can be finished in half an hour. This process, simple as it appears, requires certain precautions in using a strongly sized paper for the prints, and a very transparent original to obtain the most successful results.

It is capable of the same applications as the cyanotype last described, and, like it, is quite unsuitable for producing maps for publication.

Another process of Poitevin's is dependent on the property possessed by the ferric salts of rendering gelatine insoluble, the solubility being, however, restored when the ferric salt is decomposed by the action of light into the ferrous salt.

Paper is thinly coated with a 6 per cent solution of coloured gelatine and when dry immersed in a solution of—

and dried in the dark. After exposure to light under a positive, such as a map on tracing paper, the print is immersed in hot water, and the gelatine in the parts exposed to light dissolves out, leaving an exact transcript of the original drawing with dark lines on a white ground. The print may be rinsed in water acidulated with hydrochloric acid to remove the iron salt.

Salmon and Garnier have taken advantage of the fact that if paper is coated with a solution of the percitrate of iron and exposed to the light, the parts exposed to the light become hygroscopic in inverse proportion to the intensity of the action of light, and therefore if such paper is exposed to light under a map or drawing on thin paper or vellum cloth, and afterwards brushed over with a fine powder, such as lamp-black or plumbago, more or less of the powder will adhere to the parts protected from the light, while the exposed parts will scarcely take it at all. After development the print has only to be washed to remove the unaltered iron salt from the film.

Other similar processes of printing with the salts of iron, uranium &c., will be found in the text-books. They are, however, very little used and may be regarded more as curiosities than as practical printing methods.

Platinum printing process.-There is, however, one process which deserves mention as producing very beautiful and permanent prints, in which the image is formed of reduced platinum. This process has been patented by the inventor, Mr. W. Willis, junior. Paper is floated on a weak solution of nitrate of silver and dried. It is then brushed over with a solution of double oxalate of potassium and iron, together with a solution of chloroplatinite of potassium. After exposure under a negative the print is floated on a warm solution of oxalate of potash, which causes the platinum

* Boivin, in Moniteur de la Photographie, 1st April, 1878.

salt to be reduced in the parts exposed to the light. The prints are fixed first with hyposulphite of soda, and then with oxalate of potash and finally washed with water.

Collo-chromate printing.—We now come to the more important processes depending on the reaction of the salts of chromium, particularly the alkaline bichromates, on gelatine, gum, albumen and other colloid substances under the influence of light, whereby these substances become more or less insoluble in and unabsorbent of water in proportion to the amount of the action of light, and further acquire the property of taking up greasy ink and not attracting plumbago or other fine dry powder, also in proportion to the amount of the action of light upon them.

This simple reaction, only partially discovered in 1839 by Mungo Ponton, was first worked out and turned to practical account, some twelve years afterwards, by Fox Talbot in his process of photoglyphic engraving ; and after him Pretsch and, notably, Poitevin employed it in processes which have been the foundation of nearly all the modern methods of permanent photographic printing.

The simplest of all these processes, and one which may render useful service in the cases already noticed where only a few copies are required, was one of the first published by Poitevin. It consists in coating paper with a mixture of albumen, gum, or gelatine and bichromate of potash, coloured with Indian ink or any other suitable pigment; or, if preferred, the paper may be coated with coloured gelatine and then made sensitive in a separate bath of bichromate of potash, and this is sometimes the best method, because the paper will not keep good for long in its sensitive state. The sensitive coloured paper is exposed under a very clear line negative in a copying frame for a few minutes, and then taken out and plunged into water, either hot or cold, according as gelatine, gum or albumen have been used. The unaltered colloid in the lights of the print, which have been protected from the light under the dark parts of the negative, dissolves in the water, leaving a clear image in pigment on a white ground.

This simple method is capable of extensive use in copying maps or topographical sketches, but is only applicable to subjects in line, well drawn in black and white in accordance with the rules in Sect. III. These prints have the advantage of being quite permanent and, as the collo-chromate mixture is more sensitive to light than the chloride of silver, they can be produced at a quicker rate than the silver prints, and are, of course, cheaper on account of the inexpensiveness of the materials used.

For reproducing subjects in half tones a different procedure must be followed. In the process just described the exposure to light and the development of the print by washing are effected on the coloured side of the paper, and as the light can act with full power through the clear spaces on