Having glanced hastily at the history of bacteriology, we may next consider some facts concerning the germs themselves. What are they? To the lay mind the words germ, microbe, bacterium, and bacillus often convey confused ideas of invisible, wriggling, worm-like creatures, enemies of mankind, ever on the watch to gain a stealthy entrance into our bodies, where they wreak harm and death. Scientifically considered, however, they are the smallest of living things yet known. They are not animals, but are members of the vegetable kingdom, and are possessed of definite yet varying shapes. They consist of a jelly-like substance called protoplasm, which is covered in and held in place by a well-formed membrane of a relatively hard and dense character, exactly similar in composition to the woody fibre of trees. According to their shape the bacteria are divided into three chief groups, called respectively cocci, bacilli, and spirilla. The cocci are spherical bodies and may exist singly or in pairs, in fours, in clusters, or in chains. In this group we find the smallest bacteria known, many of them not over 1-150,000 of an inch in diameter. The bacilli are rod-like bodies, varying much in size in different species and in members of the same species. They are larger than the cocci, measuring in length from 1-25,000 of an inch to 1-4000, and in breadth from 1-125,000 to 1-16,000 of an inch. Many varieties are possessed of organs of locomotion called flagella. The spirilla resemble the bacilli, except that they are twisted into corkscrew shapes, or have gently undulating outlines. Upon an average they are much longer than the bacilli, one species being very long, measuring about 1-600 of an inch. As seen in the natural state bacteria are found to be colorless, but it is by the application of various aniline dyes that they are usually studied. These minute plants increase by a simple method of division into two equal parts, or by a more complex process of forming a seed—the so-called spore—which later on develops into the adult form. Under favorable conditions they are able to multiply at an enormous rate; for instance, it has been calculated that a bacillus dividing once every hour would at the end of twenty-four hours have increased to seventeen millions; and if the division continued at the same rate we should find at the end of the third day an incalculable number of billions, whose weight would be nearly seven thousand five hundred tons! But, fortunately for our welfare, nature by various means renders the possibility of such a happening entirely beyond the slightest chance of realization, her greatest barrier being the lack of an adequate food supply.