96 PATHOGENIC BACTERIA. cent, solution of hydrochloric acid for a few seconds. The section is subsequently dehydrated in alcohol, cleared up in xylol, and mounted in balsam, l—-- Pfeiffer's Method.—The sections are stained for one- half hour in diluted Ziehl's carbol-fuchsin (q. z/.), then transferred to absolute alcohol made feebly acid with acetic acid. The sections must be carefully watched, and as soon as the original, almost black-reel color gives place to a red violet color the section is removed to xylol, where it is cleared preparatory to mounting in balsam. For ordinary work the following simple method is recommended: After the sections are cut the paraffin must be, and the celloidin had better be, removed. From water the sections are placed in the same watery stain used for cover-glasses and allowed to remain five to eight minutes. They are next washed in water for several minutes, then decolorized in 0.5-1 per cent, acetic-acid solution. The acid removes the stain from the tissues, and ultimately from the bacteria as well, so that one must watch carefully, and as soon as the color almost disappears from the sections remove them to absolute alcohol. At this point the process may be interrupted to allow the tissue-elements to be counter- stained with alum carmin or any stain not requiring acid for differentiation, after which the sections are dehydrated in absolute alcohol, cleared in xylol, and mounted in Canada balsam. As will be mentioned hereafter, certain of the bacteria which occur in tissue do not allow of the ready penetra- tion of the color. For such forms a more intense stain must be employed. One of the best of these stains, which can be employed by the given method both for cover-glasses and tissues, is Loffler's alkaline methylene blue : Saturated alcoholic solution of methylene blue, 30; i: 10,000 aqueous solution of caustic potash, 100.