Page:Paper and Its Uses.djvu/16

4 of caustic soda and sulphate of soda (sulphate pulp), or with bi-sulphite of lime (sulphite pulp). Mechanical wood pulp is obtained by cutting the logs into short lengths, removing the bark, and grinding to pulp on a large grindstone, the surface of which is freely supplied with water. The water renders the reduction to pulp an easier matter, and also serves to carry the pulp forward for subsequent treatment.

In speaking of paper it is usual to refer to the material from which it is made, e.g., rag, rope, esparto, manilla or wood papers. It does not always follow that the papers are composed entirely of those fibres, but the prefix of "pure" or "all" will generally indicate the genuine articles. Paper which is free from mechanical wood pulp is sometimes referred to as "free."

The strongest and best papers are made from cotton or linen rag fibres. Linen is made from the fibres of the flax. In the raw state the fibres are from 1 inch to 1 inches in length, and less than of an inch in diameter. Examined microscopically the fibres appear to be smooth, cylindrical, with markings like the joints of a cane, slight cross markings, and a very fine channel running through the fibres. Papers made from linen are close, strong, and durable. Cotton is the seed-hair of the cotton plant, having a length of 1 to 1 inches with the diameter about the same as that of linen fibres. Cotton is tubular, has a large channel, and on drying the tube collapses and twists upon itself, as many as 300 twists in the length of a single fibre having been observed. This twisting assists in keeping the spun cotton together, and also makes the felting of the subsequent paper more efficient. Papers made from cotton are softer than those made from linen, and the tenderest rags, such as worn muslins, are employed for blotting papers. Hemp is obtained from