SUMMARY OF THE MORE IMPORTANT PRINTING PROCESSES WITH CHROMATED GELATINE. These processes are all based upon the change in the hygroscopic properties of chromated gelatine brought about by the action of light, which renders the parts exposed insoluble. =Carbon or Pigment Printing.=—In this interesting process finely ground colouring matter is added to the solution of chromated gelatine, and spread upon paper; or the paper may be coated with the gelatine and pigment, and afterwards excited by immersion in a solution or bath of one of the bichromates. If, after drying such paper or tissue, it be exposed under a negative in an ordinary printing frame, and then immersed in warm water (90° to 130° F.), the water will dissolve only such portions as may not have been affected or hardened by the action of light through the negative, while the parts that have been exposed to its action will be insoluble, and remain attached to the paper support, forming a positive picture. Images produced in such a manner, however, are devoid of half-tone, and this procedure is only suitable for line subjects, while even for such the paper must have been very evenly coated, or portions of the picture may be washed away. For the production of half-tone subjects it is necessary that the picture be developed _from the back_. This may be effected by transferring the printed tissue, after first soaking in cold water, to a surface, usually either of paper or glass, and developing it in contact with, and allowing it to remain upon, such surface. This procedure is known in carbon printing as single transfer. Unless the negative be a reversed one—or a film printed from the reverse side—the resulting print will be subject to the disadvantage of being reversed as regards left and right. In cases where ordinary negatives have to be used, and this reversal cannot be tolerated, it is overcome by the “double transfer” method of printing, the print in such cases being developed upon a so-called “temporary support,” and again transferred to its “final support,” when it appears as a non-reversed image in permanent pigment.[D] [D] For further details of this interesting process see the “Autotype Manual,” by J. R. Sawyer; or Liesegang’s “Carbon Process.”