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Jan., 1944 REVIEW OF HYPO TESTING METHODS 45 solution or one part in 2,000,000 parts. This is Vio of the quantity stated by Crabtree and Ross, but the lower quantity has been found easily detectable particularly with light of sufficient intensity to obtain the required light scatter when viewing against a black background or with nephelometer-type instruments. The mercuric chloride test for hypo in film is quantitative and has been adopted by the National Bureau of Standards as a standard test for hypo in films intended for archival storage. 17 Two semi-micro adaptations of the Crabtree-Ross test have been published. Gibson and Weber 18 of the National Bureau of Standards test a small punched sample of film in 2 drops of the reagent and Evans 19 of the Department of Agriculture treats Vio of a square inch of film in 1 cc of the reagent. In the latter case the opalescent test solution is picked up by a dropper-type absorption cell and a dial reading of the hypo content obtained directly by means of an apparatus built on the photoelectric principle. Soon after the recommendation of the mercuric chloride test for the estimation of residual hypo in film, the test was used by the authors for a similar purpose with paper prints. Townsend 20 has also described its use for prints. However, as stated previously, the baryta coating and paper base of prints retain an appreciable quantity of hypo which does not diffuse from the print completely. A comparison of the data obtained in hypo analyses with the mercuric chloride test (a diffusion method) and a quantitative silver nitrate test indicated that the mercuric chloride reagent determined only the readily diffusible hypo. Therefore, while this test is quantitative for film, it is not quantitative for the residual hypo in photographic prints. (d) The Silver Nitrate Test In 1908 and 1913 Lumiere and Seyewetz 21 treated films, plates, or prints with silver nitrate instead of the wash water or drippings from the material. They placed a drop of silver nitrate solution on the edge of a print, and if there was no brown coloration within 2 or 3 min, the print was considered washed. Liippo-Cramer 22 in 1912 employed the reagent as a spot test for hypo in plates and since 1930 the authors have employed similar methods of testing. Weyde 23 and Freytag 24 have suggested the use of the reagent in the following manner to determine the residual hypo in prints: