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Jan., 1944 REVIEW OF HYPO ^TESTING METHODS 43 analyses indicate that a print washed under given conditions con- tained 0.15 mg per sq in. of hypo and this is the desired degree of washing, then the strength of the iodine test solution and the time of reaction should be adjusted to give a negative test with this print. In a similar manner, a series of prints may be washed to contain quantities of hypo which cause the spot test reagent to decolorize over a range of times. After the quantity of residual hypo in the prints is determined, they are plotted on one axis of graph paper and the corresponding decolorizing times on the other axis to obtain a curve from which the hypo content of a print may be determined at different stages of washing from the time required to decolorize the reagent This curve, however, will be useful only to the operator who developed it because of the large number of vari- ables discussed above. When the spot test has been developed in this manner, the analytical results obtained may be said to be semi-quantitative. (2) Bathing Tests. —Tests may be made with iodine-azide-starch and with iodine-starch in which a film or print sample is immersed in a definite volume of the test solution for a certain time. If the blue starch-iodide has not decolorized, the material is considered washed. A bathing test may be made as follows: Place a 4 X 5-in. print or equivalent in the 125 cc of the diluted test solution contained in a 4 X 5-in. tray. The tray is rocked, and if decolorizing occurs within 2 min, the washing is considered incomplete. Dry prints may be tested by this means, but a much longer time must be allowed for sufficient diffusion. A total time of the order of 5 min in this case is required. So far as could be ascertained the testing of hypo by placing the prints in the solution had no detrimental effect on their per- manency. This method of testing permits good control of the mechanical operations, such as volume of test solution and area of material. The technique, however, is not very feasible for testing the residual hypo in enlargements. Since investigation has shown that only a part of the residual hypo is tested in this manner, it is important that the above bathing technique should be standardized by making quantitative analyses as described above and the time of bathing and strength of the test