Some species of bacteria have, in addition to the power of reproduction by simple fissure, a second method by means of spores which develop within them, and the manner in which the spores grow serves as one of the points by which the different species of bacteria are distinguished from one another. The spore serves the purpose of keeping the species alive under conditions of adversity, through its wonderful power of resisting extreme heat (in some cases for a short time, even above that of boiling water), severe cold, desiccation, and all sorts of rough usage, which would speedily destroy the parent germ. Indeed they are, says Frankland, "the hardiest forms of living matter which science has yet revealed."

*See Frankland and Conn, from whom the facts in some of these passages relative to bacteria have been taken, and even in part, in a few cases, the wording itself, to some extent interspersed with my own.

Some, but by no means all, species of bacteria have the power of active movement, the study of which forms one of the most fascinating microscopic spectacles which exist. "The varied motion," says Frankland, "of the countless swarms of individuals following their sinuous paths across the field of the microscope, in all directions, and in the three dimensions of space, much after the fashion of a cloud of midges playing in the sunshine, produces an irresistible impression upon the observer, that each individual microbe is assisting in and conscientiously performing its part in a highly complex and thoroughly organized Scotch reel, conducted at express speed." This motion is supposed to be produced by flagella, which lash the liquid with great activity and give the plants so strongly the effect of animals that it is almost impossible to believe what the authorities tell us of their being among the lowest forms of simple vegetable life similar to the oscillaria or green thread-like plants known to the botanists.

Before studying the action of bacteria in their role as sanitarians and fertilizers of the soil, it is important to say a word as to their use in the arts. Many important industries are now known to be absolutely dependent upon them as agents of fermentation and decomposition. Sometimes they split up the molecules of the substances upon which they act into simpler molecules and sometimes they build them up into more complex ones, in all cases changing their chemical natures, and thereby forming numerous useful products which could not exist without them. The world is only beginning to realize their tremendous usefulness in these ways.

There are, in the first place, many industries which may be classified as maceration industries, or based on the decomposition powers of bacteria. Hardly any organic substance is able to resist their softening influence, and man has taken advantage of this power in many arts. Thus linen, jute, hemp, and other vegetable fibrous growths require fermentation to free the valuable fibers from the woody parts with which they are associated in nature. Aided by moisture and the proper temperature the bacteria soften the fibers and thereby permit of the separation of the useful from the useless parts. This so-called "retting" process is not allowed to continue long enough for the bacteria to injure materially the valuable fibers, but only long enough to facilitate their dissolution from the rest.

The preparation of sponges for use is also accomplished by bacteria. They decompose the soft tissues constituting the body of this marine animal so that it can be removed by washing from the skeleton, which forms the sponge of commerce. Leather is prepared for the tannery by bacteria. The hide with the hair on it is steeped in warm water until partial decomposition enables the hair to be easily removed with a knife. The manufacture of citric, lactic, butyric acid, vinegars, indigo, tobacco, opium and many other substances is accomplished by the aid of bacteria in producing fermentation. In all of these processes a different species of bacterium does the work, the proper kind appearing, by some wonderful provision of nature, immediately when the proper conditions are provided for it. They seem to be ever on the alert to serve us, and hasten to the spot where needed and there multiply with marvelous rapidity until the number of workers required to accomplish the desired result in the best manner and shortest time possible has appeared.

Butter and cheese making can only be accomplished by the aid of bacteria of various kinds, and it is probable that any desired flavor will be produced by the scientific dairyman of the future by cultivating and introducing the special kinds of bacteria which experience teaches will yield the desired result.

We come now to the species of bacteria most interesting to the sanitary engineer, to those which provide plants and animals with food during life and take care of them after death. Upon these all life on the globe is dependent and would cease if their labors were suspended for any considerable length of time.

Plants and animals both require food, but while animals can live upon plants, plants are unable to obtain their entire nourishment directly from animals or other plants. Their elements must first be taken apart in order to provide food simple enough for continuing their life, and these simple products must be restored to the earth, which would otherwise soon be exhausted of its plant food instead of remaining year after year for untold ages as fertilizers. All this is done by bacteria. Moisture alone is not able to disintegrate the hard trunks of trees and the bodies of animals when they die. It could not soften their tissues and convert them to gases and elements suitable for plant food. Were it not for bacteria, which alone possess this power, the earth would soon be completely covered with dead bodies, leaving no possible room for further growth of plants and animals. This bacterial action is what is known as decay, and it is called "decomposition" when it takes place in the presence of oxygen, and "putrefaction" when oxygen is debarred. Sanitary engineers have just begun to learn the vast difference between these two forms of decay and to put their knowledge to practical use in their systems of sewerage and plumbing. They have learned that entirely different species of bacteria are employed in these two processes, and that upon their success in cultivating the one or the other depends the value of their efforts.