5.—22

4

the white salt of mercury and silver. When taken out it is well washed, and a weak solution of iodide of potassium in water poured on and off until the film is yellow right through to the glass.* As a rule the negative is now flooded with a solution of gum arabic and stood on a rack to dry ; but if considered advisable, a weak solution of chloride of gold is poured over, which increases the density. If there appears a tendency on the part of any of the fine lines to fill up, or fog, a weak solution of cyanide may be poured on and off after the treatment with the iodide of potassium. It is found in practice that the gum arabic affords sufficient protection in printing, and enables the glasses to be used over and over again without the difficulties in cleaning which would occur were the negatives varnished. The Production of the Photographic Transfer. Various ways are practised to obtain from the negative, produced as above, a positive in reverse upon the lithographic stone ; but one of two methods is mostly used, the chemical theory of both being alike, while in their mechanical effect they are quite different. In one, the white parts of the subject are transferred to the stone, leaving the lines open to absorb the greasy ink poured on them; in the other, and more frequently used method, the lines in greasy ink are transferred direct to stone. The latter is the method practised in this office, and the theory of its working may be easily understood. Gelatine, and many other organic substances readily soluble in water, if mixed when in solution with bi-chromate of potassium, acquire the property, when dry, of becoming insoluble after exposure for a short time to white light, f If paper coated with gelatine and bi-chromate of potassium, and dried, is placed in contact with a negative and exposed to the sun's rays, the action of the light through the transparent portions of the negative, which represent the lines of the subject, renders those parts insoluble ; whereas the remainder, which has been protected from the light by the opaque portions «of the negative, is in no way altered. If now the whole surface of the print be coated with transfer ink, and afterwards the print itself placed in warm water, the gelatine in the parts unaffected by light readily dissolve, and carries with it the ink, leaving the lines only upon the clear paper. This method is "that practised at the Ordnance Survey Office at Southampton, but is not adopted by me for two reasons. First, that from the fact of there being no gelatine left on the paper, except that covered by ink, it is impossible, in transferring, to pass the stone more than once through the press, for fear of the transfer shifting and the lines doubling ; and secondly, in washing away the unaltered gelatine, that which is left on the lines is apt to be ragged and broken. Osbourne's process, as practised in Melbourne, obviates these difficulties in an ingenious manner. There the gelatine is mixed with albumen, and after the print is coated with transfer ink, it is floated for a few minutes on boiling water, which coagulates the albumen, and holds it and the gelatine, or the greater portion of it, fast to the paper, the ink washing away with the aid of a sponge. This method was practised here at the commencement of work, but is now discarded. It was found that the ink, softened by the action of the boiling water, gathered to a considerable extent upon the lines, rendering them liable to " smash "in transferring, especially when used fresh; and also that soluble gelatine was present in the transfers, and required more boiling water to dissolve and clear it away lhan could be conveniently used, considering the amount of work to be performed. After a number of experiments I succeeded in working out a modification of the process, which works satisfactorily, and the following is the method in detail:— Gelatine is soaked, and afterwards dissolved in water over a gas stove, and heated to a temperature 110° Fahr., when a solution of chrome alumj is added, well stirred, and the whole strained. Paper is coated with this at a temperature of about 90° and is hung up to dry, when it becomes insoluble although still capable of absorbing water. This operation is conducted in daylight, and any quantity of paper may be prepared at a time, as it appears to keep for any length of time. Paper prepared thus has been sent by me to England, and there yielded excellent transfers. It is sensitized by floating for a few minutes on a saturated solution of bi-chromate of potash, and hung up to dry in a dark room. When dry, I claim that this sensitized paper will keep for a greater length of time without deterioration than that prepared by any other process with which lam acquainted. Tannin may be used in lieu of chrome alum, but the paper, when the latter is employed, seems to keep longer after sensitizing. A sun print is taken on this paper in the ordinary manner from the negative, and is afterwards laid face downwards on a lithographic stone coated with greasy transfer ink, and passed through the press. The surface becomes by this treatment coated with a thin coating of ink. It is next floated for a minute or two, face upwards, on a dish of cold water, and then laid upon a sheet of zinc. The water having been absorbed through the back of the paper by those portions of the gelatine not acted upon by light, the ink is readily removed with a wet sponge and a little friction. A few minutes soaking in a dish of clean cold water removes the now useless bi-chromate, and the transfer, being of a clear white, enables any remaining ink to be seen and easily removed. It is now hung up to dry, and is ready for the stone. It will be seen that the white portions of the transfer, beiug still covered with the original coating of insoluble gelatine, are yet capable of absorbing a sufficient quantity of water to render them " tacky," when wetted at the back or placed for a few minutes between sheets of damp blotting-paper, to adhere firmly to the stone in transferring; and this is of great advantage to the printer, as he is not

* Instead of the iodide of potassium, a weak solution of hydrosulphate of ammonia may be used, -which converts the deposits into sulphide of silver, but the continual stench of sulphuretted hydrogen in the place is trying to health and temper, and is therefore discarded in favour of the less offensive iodide of potassium. t " Bi-chromate of potash contains chromic acid, which is a high oxide of the metal chromium. Light alone produces no effect upon chromic acid, but if organic substances are also present the chromic acid is reduced to the condition of a lower oxide of chromium, and the liberated oxygen unites with the organic body. Some kinds of organic matter act more decidedly than others, and especially gelatine, which reduces the bi-chromate rapidly in presence of sunlight, the gelatine itself being oxidised into a resinous substance which remains in union with oxide of chromium."— Kardwich. X The proportion varies according to the quality of the gelatine, and I have found different samples of chrome alulE affect the gelatine differently.