The physical tests are more familiar to most persons, and include (1) weight per ream, (2) thickness, (3) bursting strength, (4) tensile strength, (5) folding endurance, (6) absorption, (7) expansion. 1.--There are two kinds of paper-scales. The most common kind gives, directly, the ream weight from weighing a single sheet, and is of such convenience that almost all paper-users could well afford to have one. Sensitive paper-scales for small samples, 4 by 4 inches in size, are of great assistance also, and should form part of the equipment of every paper-dealer. 2.--The thickness is determined by a micrometer gauge measuring to one-thousandth of an inch. In gauging thin papers it will prove more accurate to take four thicknesses, as the error in reading is thus quartered. The following table of bulks, which shows the number of pages per inch from a gauge of four sheets, will be found convenient: ======================+========== Thickness of four | Number of sheets in thousandths | pages to | one inch. ----------------------+---------- 8 | 1,000 8½ | 941 9 | 889 9½ | 842 10 | 800 10½ | 762 11 | 727 11½ | 696 12 | 667 12½ | 640 13 | 615 13½ | 593 14 | 571 14½ | 552 15 | 533 15½ | 516 16 | 500 16½ | 485 17 | 471 17½ | 457 18 | 444 18½ | 432 19 | 421 19½ | 410 20 | 400 20½ | 390 21 | 381 21½ | 372 22 | 364 22½ | 356 23 | 348 23½ | 340 24 | 333 24½ | 326 25 | 320 25½ | 314 26 | 308 26½ | 302 27 | 296 27½ | 291 28 | 286 28½ | 281 29 | 276 29½ | 271 30 | 267 30½ | 262 ----------------------+---------- 3.--Bursting strength is determined by a variety of testing-machines, constructed so as to record the pressure per square inch which may be exerted before rupturing the paper.