It is on this fact that the entire method of the new process is based. It is termed chemical printing with perfect propriety, as the reason why a fatty color, say, linseed oil varnish, will adhere only on the designed parts of the plate and is repelled by the rest of the surface, is due to the chemical properties of the materials. It might be maintained that in the other forms of printing, color adheres from the same reason. This is true, to be sure; for it is a general law that water and oil will adhere to all bodies that are dry. But it is not the case with these fluids mutually; and in this fact lies the unique difference between the older and the new processes. A dry plate would take color over its entire surface. If, however, it is dampened, it will take oil color only on those places that are in a condition opposite to dampness. Therefore, the repelling of the color from those parts that are to remain white is the novelty. It must not be imagined, however, that to print chemically it suffices to dampen certain parts of the plate and to coat others with fatty substance. With most of the materials available for printing, mere water does not suffice to produce a sufficiently repelling obstacle between the plate and the color. With flinty and clayey bodies,--for example, glass, porcelain, slate, etc.,--one can manage with mere water; but then the slight adherence of the fatty color to the plate produces an opposite difficulty, by preventing any large number of impressions. Still, by using very firm and readily drying fatty substances, such as linseed oil varnish dried with litharge of silver it is possible, in case of need, to succeed fairly well. But with such bodies as attract the color powerfully, such as all metals, wood, limestone, artificial stone-paper, etc., it is necessary so to treat all the parts of the plate that are to remain white that they attain an especial resistance to color, and thus change their natures, so to speak. That this is possible under certain circumstances and with the proper means, with all bodies belonging to this class, I have proved by many experiments, and I shall describe the methods in this book. Thus the new process is not to be used only on limestone, but is applicable to metal, etc.; and stone-printing or lithography is to be considered only as a branch of general chemical printing. However, as this book is to teach mainly lithography, I will occupy myself chiefly with it.