In a government bulletin, Report No. 89, United States Department of Agriculture, the following criticisms of this test are made: “This pressure is generally believed to represent the mean strength of the paper--that is, an average of the strength across and with the sheet. This is not true however, experience indicating that strength as thus determined more nearly agrees with the strength of the paper in the cross direction, with the minimum strength rather than with the average strength of the paper. “Among other objections to testers of this type, is that to a certain extent the operator can influence the results at will, and even with the greatest care there is quite a wide difference between different tests of the same paper.” 4.--Tensile strength is determined by clamping a strip of paper of standard dimensions in a machine which exerts a uniform tension until the strip breaks. The breaking strength is shown on the recorder, and the amount of stretch before breaking is also registered, thus indicating the elasticity of the paper. The best known instrument of this sort is the “Schöpper,” but the machine is very costly, hence is rarely found except in well-equipped laboratories. 5.--Folding endurance is determined on a machine which folds a strip of paper back and forth in a slot, the strip being clamped at either end to a spring device which maintains a uniform tension. The number of double folds which the strip withstands is automatically registered. This test is favorably regarded as an indicator of durability, but the apparatus is expensive and not easily available, hence this test fails of frequent use. 6.--The absorption tests are applied principally to blotting-paper, and consist in suspending equal widths of paper so their ends are submerged in a beaker of colored water. The height the water rises in a given time demonstrates the capillary attraction. 7.--Expansion is estimated by taking strips of uniform dimension, dipping in water and measuring the expansion. Chemical tests are for the determination of (1) the percentage of mineral filler; (2) the percentage and nature of sizing materials; (3) qualitative test for starch, acid, sulphur, chlorine, glue, filler material, dyes, ground wood. The amount of filler may be determined by incinerating a piece of paper of known weight. As the filler is non-combustible, the weight of the ash determines the percentage of filler, although allowance must be made for the amount of water of crystallization driven off from the mineral. Tests for acids are important in papers used for mounting tarnishable substances, such as jewelry. Tests for sulphur or chlorine are important in determining the chemical purity of the paper, since such residues militate against the permanency of color and strength of paper.