minute organisms. They referred the forms which they described to the animal kingdom, classifying them among the Infusoria, and placing a large number of them under the general title of Vibronia. Nor was this reference surprising. The means at the disposal of these early investigators were extremely crude. They saw these minute moving bodies, and considered them animal in nature: for at that time naturalists had hardly begun to realize that animals have no monopoly of motility, though it is now a commonplace observation that the simplest plants exhibit a power of locomotion that is almost wholly lost in the higher forms. They were believed to develop spontaneously. It was not until the experiments of Cohn that the doctrine of spontaneous generation as applied to bacteria was overthrown, and Harvey's law, 'Omne vivum ex vivo' ('All life comes from life') was accepted as having universal application.
Bacteria are found almost everywhere. They are present in air, water, soil, in most foods and drink, and are regular inhabitants of the mouth, stomach, and intestines, and of the superficial layers of the skin. The rancidity of butter, the putrefaction of cheese, the gamy flavor and high odor of meat, the yellowness and blueness of milk kept in imperfectly cleaned vessels, the excessive staleness of bread, and many other similar conditions in food, are largely due to the presence of bacteria. So-called bloody stains on bread, meat, paste, etc., have also been traced to the presence of a brightly colored micro-organism. In the same way the bitterness and ropiness of bad wine is brought about.
The air of the country, but especially that of cities, contains large numbers of bacteria. M. Miquel, in his laboratory at Montsouris, in Paris, constructed for the purpose of studying atmospheric germs, found that at Montsouris there were on the average about 80 bacteria to the cubic litre of air. The maximum abundance was in the autumn, the minimum in winter. The number increased at night, and was reduced by a heavy rainfall. The direction of the wind made a great difference. Thus, when the wind blew from the mountains, very few bacteria were found in the air; while, when the wind came from the direction of the city, the number was greatly increased.
Water, even that which is usually considered pure, contains bacteria in abundance. This is true of spring-water and of water from artesian wells. Stagnant water, and the water of sewers, of course, contain bacteria in immense numbers. The sulphurous springs of the Pyrenees contain an abundant bacterium (Beggiatoa), which accumulates sulphur in its body, and is especially abundant in the scum that floats on the surface. Some bacteria, called chromogenic (color-forming) on account of the bright pigment which they produce, sometimes occur in water, and have given rise to superstitious accounts of bloody rain. The red color of stagnant pools in autumn has been known for many years to be due to the presence of a micro-organism described by Ehrenberg as 'Ophidomonas sanguinea,' but now known as a species of Spirillum. Water serves as an excellent means for the transportation of bacteria, and many of the epidemics of infectious diseases are undoubtedly due to an infection of the water-supply.
As already noted, bacteria were at first placed in the animal kingdom. To Cohn (1853) is due the credit of first definitely proving. on the grounds of their structure and life-history, the fact that bacteria are plants. Nägeli (1857) confirmed the conclusions of Cohn, correcting certain details, and classifying bacteria not among the algæ, but as a parallel class of fungi. (See Schizomyotes.) Bacteria belong in that group of the vegetable kingdom whose members live on dead or live animal or vegetable matter. They are subdivided into two groups, according as they live on dead animal or vegetable matter, or on live animal or vegetable matter. The former are termed saprophytes, the latter parasites. The great importance of this distinction will appear when it is remembered that when bacteria are living on, i.e. destroying, dead matter, they arc doing good. They are decomposing dead animals and plants into their constituent elements, and returning them to the mineral kingdom again to furnish food for plant-life. In such way all decay and decomposition take place. But if they live on, i.e. consume, live animals or vegetables, they are doing harm in the sense of destroying life; and it is to this group that all harmful bacteria belong.
Classification. Bacteria are divided, according to their shape, into three main groups—(1) Bacilli, or rod-shaped bacteria: These may occur singly, in pairs, or in long threads or strings, but invariably have one diameter greater than the other. (2) Spirilli: These, like the bacilli, have one long diameter, but they have a distinct curved or spiral form. They may be short, so that they appear as mere commas; or they may be long, wavy threads, or may occur as very long spirals known as Spirochetæ, or short, thick spirals called Vibros. (3) Micrococci: To this group belong all spherical or nearly spherical forms. The group has many subdivisions, according to the way in which the cocci arrange themselves. They are called Streptococci when they are arranged in rows like a string of beads. When they are in bunches like grapes, they are called Staphylococci. When grouped in pairs, Biplococci; when in fours. Tetrads; when groups of 8 to 16 are arranged as cubes, they are spoken of as Sarcinæ. Further subdivisions have been made based upon some peculiar property. Thus, Zymnogenic bacteria are those concerned in the bringing about of fermentation; while, to the sub-group of fermentative bacteria which cause putrefaction, the name of Saprogenic is applied.
Structure and Characteristics. Each bacterium is a single non-nucleated cell of protoplasm, and is endowed with the properties which belong to living organisms as such. Bacteria differ from one another in size and shape. It has been estimated that a single bacterium of average size weighs not more than of a grain, and that 1500 of the rod-shaped forms placed end to end would about stretch across the head of an ordinary pin. It requires the highest powers of our microscopes to distinguish, not to say study, many of the forms. Many bacteria possess distinct powers of motion, due to the possession of long, hairlike appendages or cilia, which project from the body of the bacterium, and with a lashing motion propel it through any liquid medium in which it may be. This motility differs greatly in degree for
