To demonstrate the important part played everywhere by these microscopic organisms, Pasteur made two experiments. He first introduced into vessels air deprived of all dust. This process we shall have occasion to examine in all its details, in connection with the researches on spontaneous generation. In these vessels, containing pure air, were placed the water of yeast with sugar dissolved in it, milk, sawdust--all of which had been deprived by heat of the germs of the lower organisms. The vessels and their contents were then subjected to a temperature of twenty-five to thirty-five degrees Centigrade. In a series of parallel experiments, made under the same conditions and at the same temperature, Pasteur took no steps to prevent the germination of the little seeds of mould suspended in the air, or associated with the substances contained in the vessels, neither did he avoid other infinitely small germs of the class aérobies. After some time the air of all the vessels of the two series was submitted to analysis, when, behold, a very interesting fact! In the vessels where life had been withdrawn from the organic matters--that is to say, where there were no germs--the air still contained a large proportion of oxygen. In the vessels, on the contrary, where the microscopic organisms had been allowed to develop, the oxygen was totally absent, having been replaced by carbonic acid gas. And, further, for this absorption and total consumption of the oxygen gas a few days had sufficed; while in the vessels without microscopic life there remained, after several years, a considerable quantity of oxygen in a free state, so weak is the proportion of oxygen that the organic matters consume directly and chemically when the infinitely small organisms are absent. But can these microscopic organisms, after having decomposed or burnt up all these secondary products, be in their turn decomposed? How, cried M. Bouillaud, repeating his question, can they be destroyed or decomposed? How can their materials, which are of the same order as those of all the living creatures of the earth, be gasified and caused to return to the atmosphere? After having been charged with the transformation of others, whose business will it be to transform them?