1, Resorcin-test for free hydrochloric acid; 2, Günzburg's test for hydrochloric acid (Boston).] Strauss' Test for Lactic Acid.--This is the best test for clinical work, since it gives a rough idea of the quantity present. Strauss' instrument (Fig. 87) is essentially a separating funnel with a mark at 5 c.c. and one at 25 c.c. Fill to the 5 c.c. mark {221} with filtered stomach fluid, and to the 25 c.c. mark with ether. Shake thoroughly for ten or fifteen minutes, let stand until the ether separates, and then, by opening the stop-cock, allow the liquid to run out to the 5 c.c. mark. Fill to the 25 c.c. mark with water, and add two drops of tincture of ferric chlorid diluted 1:10. Shake gently. If 0.1 per cent. or more lactic acid be present, the water will assume a strong yellowish-green color. A pale green will appear with 0.05 per cent. [Illustration: FIG. 87.--Separatory funnel for Strauss' lactic acid test (Sahli).] * * * * * (4) Pepsin and Pepsinogen.--Pepsinogen itself has no digestive power. It is secreted by the gastric glands, and is transformed into pepsin by the action of a free acid. Although pepsin digests proteids best in the presence of free hydrochloric acid, it has a slight digestive activity in the presence of organic or combined hydrochloric acids. The amount is not influenced by neuroses or circulatory disturbances. Absence or marked diminution, therefore, indicates organic disease of the stomach. It is an important point in diagnosis between functional and organic conditions. Pepsin is rarely or never absent in the presence of free hydrochloric acid. * * * * * Test for Pepsin and Pepsinogen.--With a cork-borer cut small cylinders from the coagulated white of an egg, and cut these into discs of uniform size. The egg should be cooked very slowly, preferably over a water-bath, so that the white may be readily digestible. The discs may be preserved in glycerin, but must be washed in water before using. {222} Place a disc in each of three test-tubes. Into tube No. 1 put 10 c.c. distilled water, 5 grains pepsin, U.S.P., and 3 drops of the official dilute hydrochloric add. Into tube No. 2 put 10 c.c. filtered gastric juice. Into tube No. 3 put 10 c.c. filtered gastric juice and 3 drops dilute hydrochloric acid. Place the tubes in an incubator or warm water for three hours or longer. At intervals, observe the extent to which the egg-albumen has been digested. This is recognized by the depth to which the disc has become translucent. Tube No. 1 is used for comparison, and should show the effect of normal gastric juice. Digestion of the egg in tube No. 2 indicates the presence of both pepsin and free hydrochloric acid. When digestion fails in tube No. 2 and occurs in No. 3, pepsinogen is present, having been transformed into pepsin by the hydrochloric acid added. Should digestion fail in this tube, both pepsin and pepsinogen are absent.