Of the second kind are all copper and zinc plates, and the cotton-print process with copper plates or cylinders. In this method the lines and points to be printed are depressed, being either engraved, etched, or stamped. As is well known, the first method of printing is as follows: The letters, which are all at the same elevation and, therefore, furnish a plane surface, are inked with a leather ball, stuffed with horsehair. As the ball is so firm and elastic that it can touch only the elevated parts, these alone can take the color, which adheres because of its sticky nature. The same is true of the carved wood used in cotton-printing, with only the difference that, instead of rubbing with a leather ball, the wooden plate itself is laid on a cushion covered with the color, and then, being placed face down on the cloth, is hammered gently to produce the imprint. In copper and zinc printing the method is reversed. In order to force the color into the depressed parts, which alone are to be printed, the entire plate is coated with color, and then the elevated surface is cleansed again carefully. The cleaning rag cannot reach the depressed grooves, so that more or less color adheres to these according to their various depths. Under the powerful press, which forces the paper into all the engraved parts, this color transfers itself and thus gives the desired impression. It is evident that both methods rest on purely mechanical principles: book-printing being based on the fact that the color adheres only on those places that it can reach, and copper-plate printing depending on the fact that the color remains only in those places from which it cannot be removed by cleansing. It is different with the chemical print. This does not depend on either elevation or depression of the design. It depends on the fact that the design is coated with a preparation of such nature that afterward the printing color, which is made from a related substance, adheres because of its chemical similarity; and furthermore, because all parts of the plate that are to remain white, have been so treated that they repel the color. These two purely chemical objects are attained fully with the new process. Daily experience proves that all fatty bodies, such as oil, butter, tallow, fish oils, etc., and all such as easily dissolve in oil, like wax, resin, etc., refuse to unite with any watery substance without the aid of some third body that will bring about such union. The chief solvent for this purpose is alkali, which, under proper manipulation, always produces a sort of soap that then is soluble in water. Sometimes, to be sure, an apparent union can be produced by violent shaking or mixing, without the use of the alkali, but at the first opportunity the fatty substances separate themselves again from the watery ones.