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44 CRABTREE, EATON, AND MUEHLER Vol 42, No. 1 solution adjusted to measure the desired degree of washing as in- dicated by the quantitative data. (c) Mercuric Chloride Test A hypo test solution proposed by Crab tree and Ross 15 in 1930 for the determination of residual hypo in motion picture film is as follows : Mercuric Chloride-Potassium Bromide Hypo Test Solution Mercuric chloride 25 grams Potassium bromide 25 grams Water to 1 liter One square inch cut from the wet or dry material to be tested is placed in a small test tube or vial containing 10 cc of the hypo test solution and allowed to stand for 5 min without agitation. The tube is then shaken and the degree of opalescence or turbidity produced in the solution within 5 to 15 min is a measure of the residual hypo. The actual quantity of hypo tested is determined by com- paring the opalescence of the solution with a series of standard solutions made by the addition of known quantities of a 1:10,000 sodium thiosulfate (Na 2 S 2 O 3 -5H 2 O) solution to 10-cc volumes of the test solution. This test is dependent upon the formation of a white colloidal precipitate in the presence of an excess of test reagent. Under such conditions where the ratio of mercuric chloride to thiosulfate is greater than 3:2, the white compound produced is HgCl 2 -2H g S 16 according to the equation 3HgCl 2 + 2Na 2 S 2 O 3 + 2H 2 O ;=± HgCl 2 -2HgS -f 4NaCl + 2H 2 SO 4 However, the test solution contains potassium bromide so that the colloidal precipitate may be wholly or partially HgBr 2 -2HgS. It has been assumed that all of the hypo reacts with the reagent and diffuses from the film into the test solution. Two confirmatory tests have verified this assumption: (1) when the film was washed to just give a negative test with mercuric chloride, a negative test was obtained with an excess of 1 per cent acidified silver nitrate solution which reacts with all of the hypo in situ, and (2) when y 2 of a film sample was soaked in water to equilibrium and the solution tested with mercuric chloride, an identical value was obtained as by direct determination on the other half of the film sample. The sensitivity of the test solution is approximately 0.000005 gram or less of crystal hypo (Na 2 S 2 O 3 -5H 2 O) per 10 cc of the test