ment turns two rollers, one of which, E, is under the level of the bed with the plate, and serves to roll it in under the upper roller D. The plate is pinched very strongly between the two rollers, the moistened paper is pressed into the strokes of the engraving, and soaking out the ink, becomes impressed with a copy of the design. A copper-plate will yield 1,500 impressions before it sustains any injury from the pressure. The earliest impressions are called proofs, being the best. When the plate begins to wear, it is touched over with a graver. Five thousand impressions can be obtained from a copperplate, but 50,000 impressions may be obtained from a steel plate, as it is far more durable. Steel engravings are also used for printing on steel. The process is called siderography. A thick plate, or block, of cast steel is made hot until it is decarbonized, or converted into a very pure soft iron. The design is then engraved on it, and this is done with great ease when the plate is thus softened. The hardness of the plate is again restored, in other words, it is reconverted into steel, by exposing it alternately to fire and water. steel roller, softened in the same way as the plate was, is rolled over the surface of the engraving, and pressed upon it with so much force that the design becomes embossed upon the surface of the roller. The roller is now hardened in the same way as the plate was, and it is made to transfer the design to any number of softened steel plates by being rolled over them; these plates are hardened in turn, and used in printing the design on paper. An inconceivable number of engraved plates can thus be obtained from a single plate engraved by the hand. If the roller should be worn out in multiplying them, another roller can be made from the original plate. It is in this way that the Queen's heads used as postage and receipt stamps are printed. Only one head is engraved by the hand, but it has been multiplied in some instances 6,000 times on the surface of a roller. From this roller, sheets of heads are printed. As many as 500,000 impressions have been obtained on paper from one plate strongly engraved. Bank-notes are generally printed by this process. Siderography has led to what is called Natural Printing, by which a representation of any flat object can be printed from the object itself. A piece of delicate lace, for instance, is placed on a sheet of polished copper and a sheet of soft lead, the whole is forcibly pressed together, and thus a perfect impression of the lace is obtained on the soft lead. This may either be used in printing, or as a mould for making casts of a harder material. By this mode the leaves of plants, branches, flowers, feathers, wings of birds, and other substances, may be printed with astonishing accuracy; even the fibres of plants and bones of animals are delineated so naturally, that it is almost impossible to believe, that the representations are not the things themselves. Objects too brittle to bear pressure, it may be added, are printed by means of gutta percha; the gutta percha is poured over them in a liquid state, like plaster used in stereotyping, and when dry it forms a mould for casts from which the objects are printed. LITHOGRAPHY. Another kind of printing remains to be described. This is Lithography, which may be called printing with grease. It is usually called Chemical Printing, because the impressions are obtained, not by cutting strokes into a surface as in engraving, or raising strokes on a surface as in block and letter printing, but from a smooth surface, through the chemical affinities which certain things have for each other. A slab of calcareous slate stone is made perfectly smooth, and the artist draws on it the design which he proposes to print. He draws it either with lithographic chalk, or lithographic ink, both of which are made of tallow, virginwax, soap, shellac, and lampblack. The only difference between them is that the chalk is used dry, and the ink wet, being laid on with a brush. The drawing would disappear if it were rubbed over with a wet sponge, in consequence of the soap contained in the drawing materials; and, therefore, a weak solution of nitrous acid is poured over it, and this, neutralizing the soap, prevents it from dissolving in water. The stone is afterwards washed with a solution of gum, and after this is removed, it will always be found that the drawing has been rendered capable of resisting water. The design is now ready for printing, which is effected in this way-A few drops of water are thrown on the stone, but they only soak into those parts of the - surface which are not touched by the drawing; the drawing itself being greasy repels the water, and remains perfectly dry. A roller covered with printing ink is now rolled over the stone, but the ink being composed partly of oil, the parts of the stone wetted with water will not receive it, as oil and water will not unite. But the drawing being greasy, will receive the ink—it is grease to grease and, therefore, the ink passes from the roller to the drawing. Moistened paper is now laid on the stone, the whole is passed through a rolling-press, just in the same way as an engraving, the ink is transferred from the greased drawing to the paper, and, in fact, the drawing is printed. By continuing to wet the stone, and ink the drawing, an almost unlimited number of impressions may be obtained. Strange as it may seem, it is the grease which prints. The lampblack, used in making the lithographic chalk and ink, only serves to enable the artist to judge of the quantity of grease laid on the stone. That it does not help to print has been proved by a curious phenomenon. The design is often washed out with turpentine in the progress of printing, at least it disappears so entirely that persons unacquainted with the subject might suppose it was utterly destroyed. But in reality it is only the lampblack which has been washed away. The grease remains, though it has become invisible; and on the ink being rolled upon it, reappears uninjured. M |