strange and remarkable similarity of action, mentioned by Claude Bernard in his last notes, and now demonstrated by M. Duclaux and his pupils. The climax of these complex chemical reactions is reached in the humus, which is compared by M. Duclaux to a laboratory in ceaseless activity, into which the primary matter is continuously entering to be worked up there and transformed into new products assimilable by the plant.
Availing itself of the action of an external force, the solar light and heat, this laboratory employs as its workmen the microbes, which only are capable of carrying the complicated task to a good result. Fixers of nitrogen, for example, in the nodular formations of the leguminous plants, preparers of nitrates, and constantly producing soluble organic substances at the expense of insoluble matters, the microbes work untiringly in this vast abode of chemical transformations.
Yet more: as old as the living world, contemporaries of the earliest generations of plants, microbes have contributed in a powerful way to the constitution and formation of the geological strata. Peat, which later becomes coal, has been formed by the action of microbes; they have been the agents in the complex processes of precipitation by which the immense masses of various limestones have been formed; they have played a part in other reactions from which deposits of iron, sulphur, and most of the metals have resulted. This enumeration might be very much extended. These innumerable and strong chemical actions, ancient as some of them are, still play an immense part, which is absolutely necessary to the existence of the social medium. From the point of view solely of producer of coal and preparer of iron, the microbe justifies its claim to be an agent indispensable to the life of all society. But its function is still more complex and extended.
The chemical work of microbes is often used industrially by man. Two examples in which this is done may be taken as typical. Indigo is extracted from a plant which is cultivated chiefly in India, Japan, and Central America. The plant contains a sugar, indiglucin, which is separated by washing in warm water, and is then subjected to a special fermentation. The microbe splits it into indigotin and glucose. The indigotin, which is colorless, is oxidized, still by means of a microbial! reaction, and is transformed into blue indigo. This preparation would be impossible without these special microbial! reactions.
Another example of the chemical activity of microbes is furnished in the preparation of opium for smoking. The juice of the poppy, from which opium is derived, was till lately fermented in tubs to give it the desired qualities. Recently M. Calmette, of Saigon, discovered that this transformation was due to the Asper-
