Ordinarily t h e strength of wet paper is not a matter of importance. Most kinds of paper when wet are very weak and break or tear with t h e greatest ease. Certain kinds of paper, however, must be handled while wet, and it is essential, therefore, t h a t they have a sufficient wet strength t o withstand this service. This is true of photographic paper, especially of blue and brown print paper, of bag and wrapping, of paper textiles and t o a less extent of filter paper, and paper which is t o

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... be printed while wet. While papers may comply with t h e specified physical requirements under t h e usual conditions of temperature and humidity, they may prove entirely unsatisfactory for handling when wet. I t is a well-established fact t h a t t h e moisture content of t h e paper has a great effect on paper making a n d testing, and considerable work has been done t o determine t h e properties of paper under different temperatures and atmospheric moisture conditions. The strength of paper when, wet, however, has received little attention. Beadle and Stevens' present a n article on "The Dry and 'Wet Strengths of Paper." The exact method of carrying out t h e wet-strength test is not clearly stated and only t h e wet strength of papers used in t h e manufacture of paper yarns is dealt with. Perhaps one of t h e most severe conditions t o which paper is subjected when wet is in commercial blue and brown print developing. I n this work sometimes paper 4 2 in. wide and I O or 1 2 ft. long is handled while thoroughly wet. Much of t h e so-called blueprint paper will not withstand t h e necessary wetting without injury even though its other physical qualities may be entirely satisfactory. The determination of t h e tensile strength of t h e met paper has proved t o he t h e most satisfactory test for indicating t h e strength required t o withstand t h e washing necessary t o insure permanent prints. The wet strength is determined by breaking a strip of paper of a definite width, after it has been immersed in water a t a constant temperature for a definite period of time. A Schopper tensile strength machine, calibrated t o read from I t o 1000 g., is used. The jaws of t h e clamps should open in front, so t h e ends of t h e wet strips may be inserted without injury, and are set I O cm. apart, as a short strip of wet paper can be handled more easily. The test strips are cut 1 5 mm. wide a n d sufficiently long t o allow for clamping in t h e machine. Tests are made in both the longitudinal a n d transverse directions. The strips are placed separately in a water bath a t 70' F. for 20 min. After t h e specified time they are removed one a t a time and tested immediately. To obtain accurate results extreme care must be exercised in handling and clamping t h e wet strips t o prevent injury t o them. The wet strength in t h e transverse direction of I Chem. Xewa, 109 (1914), No. 2843, 242.