=Bacterium= is the generic name given to the micro-organisms belonging to the schizomycetes, whether a bacillus (rod-shaped), coccus (rounded), spiral-formed (spirillum or vibrio), or filamentous (leptothrix and spirochœta). All these are destitute of chlorophyll and multiply by fission.[9] They are all extremely small, the width usually not exceeding 1 µ = 1∕25000 inch. Various names are given to them, which are synonymous, thus: germs, microbes, micro-organisms, microzymes, bacteria (_singular_ bacterium). When they cause disease they are called _contagia_. They multiply rapidly, and may reach maturity in 20 to 30 minutes. One bacterium may, under favourable conditions, become 16,000,000 in 24 hours. =Methods of Examination.=—Until Koch discovered the method of cultivating bacteria on solid media, the science of bacteriology remained in its infancy, as it was impracticable to obtain pure, _i.e._ unmixed cultures of a given bacterium. Koch hit on the idea of mixing minute portions of cultivations of bacteria which were growing in liquid broth with liquefied gelatine, and then spreading the mixture on glass plates, and allowing it to solidify under cover, so that no atmospheric bacteria could contaminate the growth. When this was done, individual bacteria formed individual “colonies” scattered over the gelatine, and these could be identified by sub-culturing and other methods, for the details of which books on bacteriology must be consulted. The _food supply_ of most bacteria is vegetable or animal refuse. Some of them have a most useful purpose in nature, that of breaking down complex organic substances and reducing them to a simpler form. Thus bacteria play an essential part in purifying the soil, and in the operations in sewage tanks and on sewage farms (pages 192, 195 and 220). A thimbleful of ordinary garden soil which has received a periodical manurial dressing contains one to three million bacteria. Certain bacteria have been found to be capable of exercising an opposite effect, _i.e._ fixing the atmospheric nitrogen and building it up into the nitrogenous tissues of plants. Thus the nodules on the roots of leguminous plants consist of bacteria living in symbiosis with the protoplasm of the plant and supplying it with nitrogen in an assimilable condition. Pure cultures of these bacteria have been put on the market as _nitragin_, for enriching land poor in nitrogen. Thus a fairly complete cycle of nature is secured, and by rotation of plants (legumes alternating with other seeds), manures, especially nitrogenous manures, may be partially saved. The _souring of milk_ is caused by the _bacillus lactis_. This souring is an indispensable preliminary to the making of cheese, and the bacillus can now be used in pure culture to hasten the natural process. The peculiar aroma of good butter is due to a bacterium which has been isolated; and it can now be supplied in pure culture for butter-making, thus obviating bad butter.