In order to show our readers the importance of the study of the microbes, and the results that may be reached by following the scientific method created by Mr. Pasteur, it appears to us indispensable to give a summary of the history of these organisms. In the first place, what is a microbe? Although much employed, the word has not been well defined, and it would be easy to find several definitions of it. In its most general sense, the term microbe designates certain colorless algæ belonging to the family Bacteriaceæ, the principal forms of which are known under the name of _Micrococcus. Bacterium, Bacillus. Vibrio, /Spirillum, etc_. In order to observe these different forms of Bacteriaceæ it is only necessary to examine microscopically a drop of water in which organic matter has been macerated, when there will be seen _Micrococci_ (Fig. 2, I.)looking like spherical granules, _Bacteria_ in the form of very short rods, _Bacilli_ (Fig. 2, V.), _Vibriones_ (Fig. 2, IV.,) moving their straight or curved filaments, and _Spirilli_ (Fig. 2, VI.), rolled up spirally. These varied forms are not absolutely constant, for it often happens in the course of its existence that a species assumes different shapes, so that it is difficult to take the form of these algæ as a basis for classifying them, when all the phases of their development have not been studied. The Bacteriaceæ are reproduced with amazing rapidity. If the temperature is proper, a limpid liquid such as chicken or veal broth will, in a few hours, become turbid and contain millions of these organisms. Multiplication is effected through fission, that is to say, each globule or filament, after elongating, divides into two segments, each of which increases in its turn, to again divide into two parts, and so on (Fig. 2, I. b). But multiplication in this way only takes place when the bacteria are placed in a proper nutritive liquid; and it ceases when the liquid becomes impoverished and the conditions of life become difficult. It is at this moment that the formation of _spores_ occurs--reproductive bodies that are destined to permit the algæ to traverse, without perishing, those phases where life is impossible. The spores are small, brilliant bodies that form in the center or at the extremity of each articulation or globule of the bacterium (Fig. 2, II. l), and are set free through the breaking up of the joints. There are, therefore, two phases to be distinguished in the life of microbes--that of active life, during which they multiply with great rapidity, are most active, and cause sicknesses or fermentations, and that of retarded life, that is to say, the state, of resting spores in which the organisms are inactive and consequently harmless. It is curious to find that the resistance to the two causes of destruction is very different in the two cases.