printed by machinery from the web, on rollers or cylinders carrying the designs, under which the paper passes. ♦Surface tinting♦ Reference has been made to the process of coloring paper by mixing the colors in the engine, but wall-papers and many others are surface-tinted by being run through a color-vat. An iridescent or “rainbow” surface is given by treating the paper with a wash containing sulphates of iron and of indigo, and then exposing quickly to ammoniacal vapors. [Illustration: SORTING OF FINISHED SHEETS--PAGE 83] Mother-of-pearl paper is produced by a somewhat similar process. Glazed paper is first floated on a solution of silver, lead, or other metal, then, when dry, exposed to the vapors of sulphide of hydrogen, after which collodion is poured over the surface, producing rich and fascinating color effects. ♦Marbled paper♦ Marbled paper, used largely in binding, is prepared from a shallow bath of gum tragacanth, or goat’s-horn, upon which the workmen sprinkle from a flat brush the films of colors needed for the desired pattern. When the whole surface is covered with bands and splashes of color, the workman takes a huge comb, which he draws with a wavy motion the length of the tub. The practiced marbler will so lay the colors and manipulate the comb as to copy any desired pattern. The marbling is done by deftly laying the smooth white paper on the bath for a moment, and then removing it, when the entire film of color comes with the sheet, so that a resprinkling of the bath is necessary. In marbling the edges of the leaves of a book, the body of the book, without the covers, is so held that the edges may be quickly dipped into the bath. In this case, of course, one covering of coloring matter will marble a number of volumes. Paper is also colored, as has been noted, by passing the web through a coloring-bath. The papers briefly described in this chapter have been classified largely according to the methods of manufacture or chemical treatment, or to the purposes for which they were to be used. Another basis for classification is found in the size. In the United States, the usual writing papers of commerce are divided, according to sizes, as follows: ♦Sizes of writing paper♦ Commercial note 5 × 8 Letter 8 × 10 Flat cap 14 × 17 Crown cap 15 × 19 Demy 16 × 21 Folio post 17 × 22 Double cap 17 × 28 Medium 18 × 23 Royal 19 × 24 Super royal 20 × 28 Double demy 21 × 32 Double folio 22 × 34 Double medium 23 × 36 Imperial 23 × 31 Elephant 23 × 28 Double royal 24 × 38 Columbier 23 × 34 Atlas 26 × 33 Antiquarian 31 × 53 CHAPTER VII THE EXTENT OF THE BUSINESS IN THE UNITED STATES