To cultivate micro-organisms artificially they must be supplied with the proper nutrient material, perfectly free from pre-existing organisms. The secret of Koch’s methods greatly depends upon the possibility, in the case of starting with a mixture of micro-organisms, of being able to isolate them completely one from another, and to obtain an absolutely pure growth of each cultivable species. When sterile nutrient gelatine has been liquefied in a test-tube and inoculated with a mixture of bacteria in such a way that the individual micro-organisms are distributed throughout it, and the liquid is poured out on a glass plate and allowed to solidify, the individual bacteria, instead of moving about freely as in a liquid medium, are fixed to one spot, where they develop their own species. In this way colonies are formed, each possessing its own biological characteristics and morphological appearances (Fig. 260). To maintain individuals isolated from each other during growth, and free from contamination, it is only necessary to thin out the cultivation to protect the plates from the air, and to have facilities for examining them from time to time, and observing the characteristic microscopical appearances. The colonies on nutrient gelatine examined with a low power (Fig. 260), if micro-organisms such as _Bacillus anthraces_ and _Proteus mirabilis_, the naked eye appearances in test-tubes of the growth of the bacilli of anthrax and tubercle, and the brilliant growth of micro-coccus prodigiosus, may be given as examples in which the appearances are often very striking and sometimes quite characteristic. I must, however, first direct attention to a well-recognised fact, that bacteriology only touches animal pathology at a few points, and that so far from bacteria being synonymous with _disease germs_, the majority of these remarkable organisms appear to be beneficent rather than inimical to man. This is of immense importance to science, as I shall attempt to show further on; although even a brief description of all the useful ferments due to bacteria and brought into use would occupy a volume to themselves, and call for a school of bacteriology quite apart from that involved in the medical aspect of the question, for the purpose of fully investigating problems raised by the agriculturist, the forester, the gardener, the dairyman, brewer, dyer, tanner, and other industries, which open up vistas of practical application, and to some extent are already being taken advantage of in commerce.