Fig. 2.--_A_, Sputum showing tubercle bacilli stained with carbol-fuchsin and Gabbet's methylene-blue solution (obj. one-twelfth oil-immersion); _B_, sputum of anthracosis, showing particles of coal-dust stained with methylene-blue (obj. one-twelfth oil-immersion) (Boston).] When they are not found in suspicious cases, one of the following methods should be tried: * * * * * (1) Take a few drams of the sputum in a test-tube, add hot {35} water, and heat until the albumin is coagulated. Let settle for twenty-four hours, or centrifugalize at once, and examine the sediment for tubercle bacilli. (2) Boil the sputum with just sufficient weak caustic soda solution to render it fluid; neutralize with acetic acid; add several times its volume of water; centrifugalize; and stain the sediment, adding a little of the untreated sputum to make the smear adhere to the cover-glass. (3) Inoculate guinea-pigs. * * * * * There are a number of bacilli, called _acid-fast bacilli_, which stain in the same way as the tubercle bacillus. Of these, the smegma bacillus is the only one likely ever to cause confusion. It, or a similar bacillus, is sometimes found in the sputum of gangrene of the lung. It occurs normally about the glans penis and the clitoris, and is often present in the urine. The method of distinguishing it from the tubercle bacillus is given later (p. 127). Other bacteria than the acid-fast group are stained blue by Gabbet's method. Those most commonly found are staphylococci, streptococci, and pneumococci. Their presence in company with the tubercle bacillus constitutes _mixed infection_, which is much more serious than single infection by the tubercle bacillus. It is to be remembered, however, that a few of these bacteria may reach the sputum from the upper air-passages. Clinically, mixed infection is evidenced by fever. (2) Staphylococcus and Streptococcus (p. 262).--One or both of these organisms is commonly present in company with the tubercle bacillus in the sputum of advanced phthisis (Plate II, Fig. 2). They are often found in bronchitis, catarrhal pneumonia, and many other conditions. (3) Pneumococcus (Diplococcus of Fränkel).--The pneumococcus is the causative agent in nearly all cases {36} of croupous pneumonia, and is commonly found in large numbers in the rusty sputum of this disease. It is sometimes met with in the sputum of catarrhal pneumonia, bronchitis, and tuberculosis. It has been found in the saliva in health. Pneumococci are about the size of streptococci. They are ovoid in shape, and lie in pairs, end to end, often forming short chains. Each is surrounded by a gelatinous capsule, which is its distinctive feature (Fig. 9). Diplococci without capsules are common in the sputum, but have no special significance. [Illustration: FIG. 9.--Diplococcus pneumoniæ in the blood (Fränkel and Pfeiffer).]