BACTERIOLOGY. Though generally considered a modem science, and perhaps prop erly as regards certain of its most important developmental aspects, bacteriology in reality dates from the observations of the-Dutch inves tigator Leeuwenhoek in the latter part of the 17th century. With simple lenses ground by himself, Leeuwenhoek discovered in the mouth, in the excreta, in water and in other matters examined by him, the presence of countless bodies of smaller dimensions than Anything hitherto seen. These °animalcules," as he called them, were often observed to move themselves about in a remarkably energetic manner, and, judging froehis text and illustra tions, they were doubtless the bodies we now recognize as bacteria. Leeuwenhoek's observa tions were immediately seized upon by the philosophers of the day as offering an explana tion for many hitherto unexplained phenomena. So general became the belief in a casual rela tion between the °animalculesx' and all manner of disease conditions that for a time, we are told, there prevailed almost a °germ mania.° To the investigators of the time the question of greatest fascination in connection with this newly-discovered world was as to its origin. Many believed and stoutly maintained that the °animalcules° were the products of metamor phosis of either living or dead tissues of more highly organized beings; others that they arose de novo in °putrescent atmospheres)); . many suspected them of spontaneous generation in some other mysterious way; while a few main tained, on experimental evidence, that they were probably the descendants of pre-existing crea tures of the same kind. Singular as it may seem it took nearly two centuries to close finalbr that debate and to prove that the dictum of Harvey °mune vivum ex ovo° or better, its appropriate modification ((ompte vivum ex vivo° was as ap plicable to the microscopic as to the world of higher beings. In its modern aspect bacteriol ogy dates from the epoch-malcing investigations especially of Koch and of Pasteur conducted during the 8th decade of the 19th century. During that period observations were made and methods of work devised that went far toward starting the subject on its career as a science. In the study of bacteria, as of all other forms of life, it is essential to a correct interpretation of form and physiological function that the ob seiyations be made upon isolated species. Pnor to the period mentioned this was not possible, for the methods in vogue were insuffi cient for the separation of these minute crea tures from one another. For the development of the science probably the most important step was, therefore, the introduction by Koch of trustworthy methods for the separation of individual bacterial species from mixtures of them, and for the more or less complete de terminanon of their specific morphologicai and physiological peculiarities; that is, for the isolation and study of bacteria in °pure cultiva tion,' as it is technically called. Up to the time of Koch's classical research upon the methods of investigating bacteria, their study had been conducted in fluid materials; that is, in infu sions of either vegetable or animal matters, in which most bacterial species develop with re markable activity. Since many totally distinct
species are indistinguishable from one another by their size, shape and general appearance, it was obviously impossible, by the older methods of study, either to be certain if one were deal ing with one or more species in the fluids in which they were growing, or to separate the one from the other in case of confusion. Koch appreciated this defect and suggested the use of solid materials as culture media, hoping thereby to reproduce the conditions so often seen when such organic matters as bread, potato, cheese, etc., become moldy on exposure to air. Here one sees the mold not always as an inextricable mixture of different species, but often as sharply isolated islands of beginning growth— as mold colonies —so to spealc. These, on examination, are usually found to consist of single species, and on a slice of moistened bread one may often observe several colonies of distinct species growing side by side without, for a time at least, encroaching one upon another. By appropriate methods it is easily possible to transplant such colonies, free from admixture with other forms, and study them as °pure cultures.° But such sub stances as bread, potato, etc., are not in general as well adapted to the study of bacteria as to that of molds. Appreciating this Koch demon strated that the addition of gelatin to the in fusions that had been employed for the success ful cultivation of bacteria converted them into practically solid culture media without robbing them of any of their useful properties; and that by the appropriate employment of such solid media it was easily possible to separate as pure cultures the individual species composing the mixtures of bacteria that one desired to analyze. Thus, for example, if a tube of gela tinized beef tea, freed from all living bacteria by heat, be gently warmed until liquified, and be then inoculated with a mixture of several spe cies of bacteria, growth at once begins and if left in the test-tube progresses in about the same mariner as if the beef tea did not contain gelatin; but if while still warm and fluid the contents of the tube be poured out upon a flat, cold surface, the increased area causes the bacteria to become more widely separated from one another and the lower temperature results in the solidification of the gelatin, so that each bacterium. is fixed in its new position. It at once begins to germinate, and presently a °colony° results; the surface ultimately becom ing studded with such colonies. As the colo nies from the different species differ from one another in many ways —in outline, texture, color, effect of their growth on the gelatin, etc. —it is easily possible, after a little practice, to distinguish them by the naked eye, and by transplanting them to tubes of sterile culture media to study them without the disturbing presence of other species; that is, in pure culture.