_Blood_ is often recognized by simple inspection, but more frequently requires a chemic test. It is bright red when very fresh, and dark, resembling coffee-grounds, when older. _Particles of food_ eaten hours, or even days, previously, may be found, and indicate deficient motor power. {218} Search should always be made for _bits of tissue_ from the gastric mucous membrane or newgrowths. These, when examined by a pathologist, will sometimes render the diagnosis clear. C. CHEMIC EXAMINATION A routine chemic examination of the gastric contents involves qualitative tests for free acids, free hydrochloric acid, and organic acids, and quantitative estimations of total acidity, free hydrochloric acid, and sometimes combined hydrochloric acid. Other tests are applied when indicated. 1. Qualitative Tests.--(1) Free Acids.--The presence or absence of free acids, without reference to the kind, is easily determined by means of Congo-red. * * * * * Congo-red Test.--Take a few drops of a strong alcoholic solution of Congo-red in a test-tube, dilute with water to a strong red color, and add a few cubic centimeters of filtered gastric juice. The appearance of a _blue color_ shows the presence of some free acid (Plate IX, B, B'). Since the test is more sensitive to mineral than to organic acids, a marked reaction points to the presence of free hydrochloric acid. Thick filter-paper soaked in Congo-red solution, dried, and cut into strips may be used, but the test is much less delicate when thus applied. * * * * * (2) Free Hydrochloric Acid.--In addition to its digestive function, free hydrochloric acid is an efficient antiseptic. It prevents or retards fermentation and lactic-acid formation, and is an important means of protection against the entrance of pathogenic organisms into the body. It is never absent in health. * * * * * {219} Amidobenzol Test.--To a little of the filtered gastric juice in a test-tube, or to several drops in a porcelain dish, add a drop of 0.5 per cent. alcoholic solution of dimethylamido-azobenzol. In the presence of free hydrochloric acid there will at once appear a _cherry-red color_, varying in intensity with the amount of acid (Plate X, C). This test is very delicate; but, unfortunately, organic acids, when present in large amounts (above 0.5 per cent.), give a similar reaction. Boas' Test.--This test is less delicate than the preceding, but is more reliable, since it reacts only to free hydrochloric acid. In a porcelain dish mix a few drops of the gastric juice and the reagent, and slowly evaporate to dryness over a flame, _taking care not to scorch_. The appearance of a _rose-red color_, which fades upon cooling, shows the presence of free hydrochloric acid (Plate IX, 1).