The mixture of pulp and water is pumped into two or more vats and formed into a number of thin sheets, which are all brought together between squeezing rolls and passed through heavy press rolls which compress the several layers into a compact mass. The thick sheet obtained is dried over steam-heated cylinders which are placed at the end of the press rolls, and calendered. The whole process, indeed, resembles that of ordinary paper-making, the main difference being the method of producing the wet sheet or card. Some machines are constructed with six or seven vats and forty to fifty drying cylinders, and are capable of turning out a large quantity of finished material. The board can be made of uniform quality and texture throughout, or be finished off with high-grade paper on one or both sides. In the latter case the constituents of the "middle" part are waste papers and raw material of inferior quality, the outer surface of wood pulp, white or coloured according to circumstances. The variety of papers and boards which can be produced is due to the fact that the several vats of pulp are independent of one another and can be filled with any kind of paper stock. The combined sheets forming the ultimate board are dried on the ordinary cylinders, calendered, and reeled up at the end of the machine. CHAPTER VII SPECIAL KINDS OF PAPER There are many varieties of paper products obtained by submitting finished paper to a number of special processes. Of these only a few of the more important will be described. These products can be divided approximately into three classes:-- (1) Papers coated on one side or both sides with various substances, such as "art," photographic papers, etc. (2) Papers impregnated with chemicals, such as blue print, medicated, and cheque papers. (3) Paper pulp converted into modified products by chemical treatment, such as vulcanised board, viscoid, etc. Of the first class, the coated papers used for art and chromo illustrations are the most important. Of the second class, the blue prints and papers impregnated with chemicals, chiefly employed for the production of engineers' drawings, may be regarded as typical. In the third class, vegetable parchment and vulcanised board are the most familiar. * * * * * _Parchment Paper._--This is produced by the action of sulphuric acid upon ordinary paper, the most suitable for this purpose being made from unsized cotton rag, free from such additions as mechanical wood pulp. The presence of the latter substance should be avoided, as it is liable to char or burn, so that in the finished product it shows itself in the form of small holes. The process depends upon the power of sulphuric acid to change the surface of the paper into a gelatinous mass, which has been shown to consist of a substance called amyloid.