394. Nature Of Bacteria. The word bacteria is the name applied to very low forms of plant life of microscopic size. Thus, if hay be soaked in water for some time, and a few drops of the liquid are examined under a high power of the microscope, the water is found to be swarming with various forms of living vegetable organisms, or bacteria. These microscopic plants belong to the great fungus division, and consist of many varieties, which may be roughly divided into groups, according as they are spherical, rod-like, spiral, or otherwise in shape. Each plant consists of a mass of protoplasm surrounded by an ill-defined cell wall. The bacteria vary considerably in size. Some of the rod-shaped varieties are from 1/12,000 to 1/8,000 of an inch in length, and average about 1/50,000 of an inch in diameter. It has been calculated that a space of one cubic millimeter would contain 250,000,000 of these minute organisms, and that they would not weigh more than a milligram. Illustration: Fig. 168.—Examples of Micro-Organisms called Bacteria. (Drawn from photographs.) A, spheroidal bacteria (called _cocci_) in pairs; B, same kind of bacteria in chains; C, bacteria found in pus (grouped in masses like a bunch of grapes). [Bacteria in A, B, and C magnified about 1000 diameters]. D, bacteria found in pus (tendency to grow in the form of chains). [Magnified about 500 diameters.] Bacteria are propagated in a very simple manner. The parent cell divides into two; these two into two others, and so on. The rapidity with which these organisms multiply under favorable conditions, makes them, in some cases, most dangerous enemies. It has been calculated that if all of the organisms survived, one bacterium would lead to the production of several billions of others in twenty-four hours. 395. The Struggle of Bacteria for Existence. Like all kinds of living things, many species of bacteria are destroyed if exposed to boiling water or steam, but seem able to endure prolonged cold, far below the freezing-point. Thus ice from ponds and rivers may contain numerous germs which resume their activity when the ice is melted. Typhoid fever germs have been known to take an active and vigorous growth after they have been kept for weeks exposed in ice to a temperature below zero. The bacteria of consumption (bacillus tuberculosis) may retain their vitality for months, and then the dried expectoration of the invalids may become a source of danger to those who inhale air laden with such impurities (sec. 220 and Fig. 94). Like other living organisms, bacteria need warmth, moisture, and some chemical compound which answers for food, in order to maintain the phenomena of life. Some species grow only in contact with air, others need no more oxygen than they can obtain in the fluid or semi-fluid which they inhabit.