When a mould thus fitted with the design is dipped into a vat of pulp and lifted out, the water falls through the wire, and the pulp sinks down on to the surface of the mould, forming a replica, so to speak, of the design, which is easily seen when the dry paper is held up to the light, because the paper is thinner just at those points where the wires forming the design come into contact with the wet pulp.
Some of the watermarks are very elaborate and interesting. A familiar illustration of a beautiful design of this description is to be found in the Bank of England notes. As a general rule the ordinary watermark consists of a mere trade term such as “Vellum,” “Zenobia,” or of the name of the manufacturer, such as “J. Whatman,” “R. Batchelor,” and so on. In the earlier days of paper-making many highly interesting designs were used, and some of these are still extant. In fact many of the names by which certain standard sizes of paper are known owe their origin to the watermarks employed.
The earliest known watermark bears the date A.D. 1301, being in the form of a globe and cross, as shown. Of equal interest are those designs from which certain papers are called foolscap, crown, pott, post, royal, columbier, and so on. The watermarks are now little used, but the terms are still retained, as indicating the size of the sheet.
Microscopic Features of Cotton and Linen Fibres.
The cotton fibre is about 30 mm. long, with an average diameter of ·025 mm. of tube-like shape, and having a prominent central canal. There are no cross markings on the cell walls, and the ends of the fibre are rounded off into a somewhat blunt point. It exhibits a marked tendency to twist itself, especially if dry, and this peculiarity is readily observed with the raw material.
The process of paper-making alters the characteristic structure of the fibre very greatly. The ends of the fibre are seldom to be seen; the curious twist is less prominent, and the fibres are torn and destroyed. The effect of the beating process, for example, on cotton is easily to be noticed by comparing the fibres of a blotting paper under the microscope with the fibres of a bank or loan paper.
The distortions produced by prolonged beating renders the determination of the exact percentage of cotton in a rag paper rather difficult, but the features to be looked for are the absence of pores, cross markings, the existence of a central canal, striations produced in many cases on the cell walls parallel to the length of the fibre. The structural features are more readily observed when the fibres are stained with a suitable reagent. (See page [71].)
Fig. 18.—Cotton.