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450 THE PHOTOGRAPHIC NEWS. [July 20, 1883. has been partially washed away. In cases where there is great tendency to frill—such, for instance, as when a soft sample of gelatine has been employed, or old decomposed emulsion worked in with the fresh emulsion—it will in such cases be safer to put the plates in the normal bath for a few minutes previous to immersing them in the acid bath. Potash alum is obtained tolerably pure in commerce in colourless transparent crystalline masses, having an acid, sweetish, astringent taste. It is soluble in 18 parts of water at 60° F., and in its own weight of water at 212° F. ; but the excess crystallizes out upon cooling. The solu tion reddens litmus paper, and, when impure, usually con tains traces of oxide of iron. Upon the addition of either caustic soda or potash, a white gelatinous precipitate is formed (hydrate of alumina), which is soluble in excess of the re-agent employed. The precipitate thus obtained has much of the character of the opalescent film some times observed on gelatine plates, when dry, which have been soaked in alum, and not well washed afterwards. Alkaline carbonates—such as washing soda, for instance —precipitate hydrate of alumina, which does not dissolve in an excess of the re-agents, and carbon di-oxide is evolved. Ammonia hydrate produces a precipitate in a much finer state of division, which does not dissolve in excess when examined in a test-tube; it somewhat resembles thin starch paste. The presence of traces of iron may be known by adding a few drops of hydrochloric acid to a small quantity of a saturated solution of alum in a test-tube, to which add strong liquid ammonia ; should any iron be present, the mixture will have a reddish-brown tinge when examined over a sheet of white paper. Other alums exist, such as the double sulphate of alumina and sodium, and sodium or aluminum and ammonium ; but hitherto their uses have been confined to the experimental portion of the com munity rather than the practical. CARBON PICTURES FOR CRYSTOLEUM PAINTING. BY W. T. WILKINSON. The ancient process of colouring silver prints from the back, under the modern patronymic of crystoleum, has been the means of calling into existence various firms whose business is to supply photographs and materials to the general public, which materials have had an immense sale. The success that has attended the venture may be put down to the fact that crystoleum is art made easy, as by cementing the photograph upon a piece of glass, and by the aid of various mediums for making it transparent, the application of a little colour upon the back quickly trans forms the monochrome photograph into a highly-coloured miniature, and this result being attained without any knowledge or skill in drawing, its popularity can scarcely be wondered at. Photographers, as a body, have not given so much attention to the principle upon which this process is based as perhaps they ought to have done ; one reason, no doubt, being that the effects, although done with a reasonable amount of skill, have had a more or less pasty effect—an effect that, on account of the opaque nature of the paper support of the silver print, could not be remedied. Now the principle of the process, and no doubt the reason of its immense popularity, is, that the drawing and composition of the picture being supplied by the photo graph, and the application of the colour being upon the back, far less skill is required than when the colouring is attempted upon the front, or where it would be necessary to both draw and paint the picture. Photographers know to their cost the difficulty ex perienced (except with highly-paid artists) in keeping the likeness intact when photographs are coloured on the front; therefore, to guard against this, and at the same time get a more highly-finished picture, let the colouring be done at the back. But ordinary silver prints, in spite of all the clearing and preservative mediums, cannot be made sufficiently trans parent to enable the best skill of the colourist to be shown, nor to guarantee the permanency of the result when done; so, instead of a silver print, try one in carbon developed upon the glass (flat or concave) ; then a really first-class picture may be made upon which the colourist may lavish all his skill with the assurance that every stroke made for effect will give that effect, that the likeness will not be disturbed, and that the work is permanent. Carbon images are easily developed upon glass either flat or on the inside of the convex glasses; associated with this class of work, in either case the glass must be prepared with a substratum of bichromated gelatine, and, when dry, exposed to light, so as to render this film quite insoluble. To mount the exposed tissue upon the convex glasses the best squeegee is a piece of india-rubber about two inches square, and one-eighth of an inch thick ; but for flat glasses, the ordinary squeegee will be best; in either case, be sure and do not spare the squeegee. The best class of negative for the purpose is a very thin one—almost too thin for silver printing ; and the best tissue is the Autotype Company’s Portrait Brown No 113. To colour these pictures, use the best oil colour, stippling the hands and face, not merely putting on a complexion colour; jewellery, &c., being put in in gold or silver, and the whole backed up with white pigment, either pure or modified, with warm or cold colour, as taste dictates. Bear in mind that the carbon image is nearly transpa rent, and is there to give the drawing only, so that all the colour effect must be done thoroughly and properly, as a daub, instead of making a finished picture, will only spoil, instead of improving a good photograph. Before commencing to colour, any spots or blemishes in the print must be removed with oil colour the same tint as the carbon image, taking care to put the colour on as thin as possible, else the work may show through the colour. Full details as to the ordinary crystoleum process are to be found in the current Year-Book. FRENCH CORRESPONDENCE. Captain Biny’s TyPOGRAPHIC Process—The Photooraphic Society Of France—ENLARGEMENTS on Gelatino-Bbo- mide Paper—Steinheil’s Objective—Transit of Venus —M. Davanne’s Lectures—Photo-ceramic Lectures at Limoges—Exhibition in Vienna. Captain Biny's Process.—1My experience in various print ing processes enables me to set a high value on a new method shown by Captain Biny, whose works are well known. It is a mode of engraving on zinc to be printed from a lithographic press. Zinc must first be selected free from streaks and traces of oxidization. It is cleaned with a soft brush dipped in water acidulated with hydrochloric acid in the proportion of three per cent. Next it is washed, and plunged in a wet state into a saturated solution of iodine and gallic acid and two per cent, of liquid phosphoric acid, at the ordinary temperature. The zinc plate should bo rubbed over with a brush to prevent the formation of bubbles, and afterwards freely washed in water, and dried between blotting-paper. It may then be coated with the sensitive material (bitumen of Judea, or bichromated albu men), and exposed behind the original positive design, black and homogeneous in character. The length of expo sure is controlled by any of the various well-known methods, and the plate developed in the usual way. It is etched in fifty seconds with water acidulated with three per cent, of nitric acid. This done, the insoluble matter is peeled off, and the plate dried carefully. To ink the plate, it must bo lightly rubbed over with oil with the palm of the hand, the excess dried off, and then lithographic ink applied to the