of healthy mice from nothingness. The discovery was soon made that a piece of gauze tied over the mouth of a jar containing decaying meat prevented the development of maggots within. The mother blow-fly was even observed in the act of laying her eggs upon the surface of the gauze, having been attracted there by the odor of the putrefying mass inside. Maggots appeared when these eggs hatched and a new fact was in this way added to science. Thus, one by one, the grosser experiments designed to prove the doctrine of spontaneous generation failed in their original intent, although many new discoveries resulted therefrom. Mice, maggots, man, and life in general, were found to arise from preëxisting life of the same kind. The day of the microbe was close at hand.

One type of experiment in the field of spontaneous generation for many years baffled all attempts of the anti-spontaneous generationists to disprove it. It was observed that infusions prepared from meat or vegetables - clear, limpid, sweet, nutritious fluids, enclosed hermetically in flasks or bottles as soon as prepared - rapidly became turbid and foul-smelling. This took place even when they were actually sealed into glass vessels. An examination of the cloudy contents of these vessels under the

microscope revealed countless hordes of "animalcules," invisible or nearly invisible to the unarmed eye. Whence came these microscopic beings which developed in ever-increasing numbers in infusions that were so devoid of evidences of life and so pleasant when they were first prepared? Did these micro-organisms actually generate themselves, as the dogmatists claimed, or were the parent cells present, but unrecognized, in the nutrient fluids at the time they were enclosed in their glass prisons, as the biologists alleged?

Many ingenious experiments were made. Attack and counter-attack upon these experiments led to the discovery that the air harbors numerous microbes, too small to be seen by the naked eye, but made noticeable in a powerful beam of light directed through a darkened chamber. Some germs were encountered which would survive several hours' exposure to the temperature of boiling water. Others were found to be actually poisoned by the oxygen of the air, that element which is absolutely essential for the preservation of the life of most animals, plants, and for man. In the end the theory of spontaneous generation remained a discredited hypothesis. The final victory of the opponents of the doctrine that life actually may be shown to

make its appearance spontaneously from organic but lifeless substances required the perfection of the compound microscope, the rise of a new science Bacteriology and the labors of one of the most versatile geniuses the human race has ever known, Louis Pasteur.

Out of this conflict between empiricism and carefully executed experimentation, much good has come to mankind. There has arisen the germ theory of disease, antiseptic surgery, curative serums and antitoxins, methods for inducing resistance to microbic infection, germicides and antiseptics, all of which are of fundamental importance in communal existence. There has also arisen a gradual recognition of the extraordinarily useful and important part microbes play in the economy of nature and the conduct of everyday life. Civilization owes much to the microbe.

CHAPTER II

The forms and the appearance of bacteria Cocci, bacilli, and spirilla - The size and weight of bacteria - How bacteria grow and multiply Their rate of growth Influences restraining multiplication - How bacteria move: their rate of motion - Bacterial spores and bacterial hibernation Longevity.

1

BACTERIA, germs, or microbes, are the smallest living things with which science has definite acquaintance. They are far too small to be seen with the naked eye. Observed under the microscope with high magnification, however, they are plainly visible. When alive, they appear as minute, colorless sacs of clear substance without any differentiation into the root, the stem, and the leaf of plants, or the arms, legs, body, and head of animals. All of the vital functions of bacteria, nutrition and growth, multiplication and function, are carried out in a single sexless cell of extremely minute proportions. For this reason bacteria are called "unicellular," or one-celled living things. Certain anilin dyes color the substance of germs deeply and make them, therefore, much more readily detectable;

1 The exactly correct term for the micro-organisms discussed in this volume is "bacteria." Germ and microbe are used popularly and loosely to mean bacteria (in which sense they are to be interpreted in this work), or, less commonly, to mean a variety of micro-organisms not necessarily bacteria, as yeasts, moulds, and the like.

little or no more detail is brought out, however, unless very tedious methods of coloration are employed. Even then, with the exception of very delicate, threadlike processes extending from the body of the microbe, little or nothing additional is seen. These threadlike processes are called flagella.

THE FORM OF BACTERIA. There are many kinds of bacteria, but all fall into one of three great groups, depending upon their form or shape. Some are spherical, like minute berries. Indeed, the name of this group is derived from a Greek word (кóKKOS) meaning a berry. It is written coccus. A second group includes all bacteria which are rod-shaped. These appear under the microscope as tiny, sausage-shaped sacs, some rather long and thin, others thicker and shorter. These rod-shaped bacteria are called bacilli, from a Latin word (bacillum) meaning a stick or staff. A single individual of this kind is referred to as a bacillus. Finally, a third group is found to be elongated as the bacilli, but with this difference, that the elongated organisms are curved like a corkscrew or spring. The curvature may be very close, like a closely coiled spring, or merely a slight, sinuous twist, like half a doughnut. The curved, elongated bacteria are called spirilla from a