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.) [Illustration: FIG. 18.--Cotton.] The _linen_ fibre has an average length of 27 mm. with a diameter of ·02 mm. The raw flax is very different from raw cotton and is easily distinguished. The fibre is slender in shape, having thickened knots at regular intervals throughout its length, the general appearance of which may be compared to a stick of bamboo. The central canal of the fibre is extremely narrow, running like a small thread through the length of the fibre. The cell walls are further marked by numerous pores, which appear as small dark lines running from side to side, but not meeting in the centre. [Illustration: FIG. 19.--Linen.] In the treatment necessary for making paper these characteristics are largely destroyed, and while it is quite easy to ascertain that a paper is of linen, or of cotton, or that a paper is mainly cotton with a small percentage of linen, yet there are conditions under which it is difficult to determine the exact percentage of cotton or linen in a rag paper. If, for example, a paper contains nearly equal quantities of cotton and linen, the exact proportions cannot be determined closer than 10 per cent., especially in well-beaten papers. REAGENT FOR STAINING FIBRES.