TUBERCULOSIS. 217 a dish of twenty-four-hours-old, newly-filtered Ehrlich's solution, and allowed to remain twelve to twenty-four hours at the room-temperature or one to two hours in the incubator. From the stain they are placed in water, where they remain for about ten minutes to wash. They are next immersed in acid (20 per cent nitric acid) for about two minutes, and become greenish-black. From the acid they are placed in absolute alcohol, and are gently moved to and fro until the pale-blue color returns. They are then washed in three or four changes of clean water until they become almost colorless, and are then removed to the slide by means of a section-lifter. The water is absorbed with filter-paper, and then the slide is heated over a Bunsen burner until the section becomes shining, when it receives a drop of xylol balsam and a cover-glass. It is said that sections stained in this manner do not fade as quickly as those stained by Ehrlich's method. The tubercle bacillus also stains well by Gram's method, but as this is a general method by which many different bacteria are colored, it is ill adapted for purposes of differ- entiation, especially when the prosecution of the charac- teristic methods is not more difficult. So far as is known, the tubercle bacillus is a purely parasitic organism. It has never been found except in the bodies and excretions of animals affected with tuber- culosis, and in dusts of which these are component parts. This purely parasitic nature greatly interferes with the isolation of the organism, which cannot be grown upon the ordinary culture-media. Koch first achieved its arti- ficial cultivation by the use of blood-serum. When planted upon this medium the bacilli are first apparent to the naked eye in about two weeks, and occur in the form of small dry, whitish flakes, not unlike fragments of chalk. These slowly increase at the edges, and grad- ually form scale-like masses of small size, which under the microscope are seen to consist of tangled masses of bacilli, many of which are in a condition of involution.