Yeast cannot transform itself into lactic ferment. No matter what the medium may be in which it is sown, _if it is actually pure_ it will never present the least trace of lactic ferment or of any other ferments. By certain changes in the nature of the medium, the temperature, and other conditions, the cells of the ferment may become oval, elongated, spherical, and larger or smaller in size, but they will never produce the most minute quantity of lactic ferment or lactic acid. The whole theory of the transmutation of ferments, which M. Duval has published, is imaginary.[24] We have asserted that the source from which we obtain our supply of the substances that we expose to contact with the air may likewise furnish a reason for the development of vegetable or animal organisms which make their appearance in our infusions. This may be easily understood. If we expose the infusions to temperatures more or less high, with the object of destroying the vitality of the germs which they may contain, we completely suppress all germs originating from two different sources—those which the infusions may have acquired directly from the atmospheric air, or from the dust upon our utensils, and those which may have been introduced by the materials used in manufacturing our infusions or decoctions, which materials must have been brought from some distance. An infusion, after having been heated to a sufficient degree, can harbour and nourish only such germs as are conveyed to it by the air after the heating. These floating germs are far from being as varied in their nature as it pleases those to believe who are tied down by their arbitrary and faulty interpretation of the knowledge that we have acquired, and the discussions in which we have taken part, during the last fifteen years. Air, unless in violent agitation, can hold only the most minute particles in suspension. The observations which have been recorded concerning organisms of spontaneous growth found in infusions, have always been made in sheltered places—in rooms or laboratories, the atmosphere of which is, relatively speaking, very still. For this reason, in liquids that have been subjected to heat, the _flora_ and _fauna_—if we may use such an expression—are very poor, and all the more so because, as we have recently had occasion to remark, a great number of the organisms which would spring into existence of their own accord, if they were allowed sufficient time for germination, are kept back by others of a more rapid development. The truth of this is evident from the fact that we may observe greater variations in the nature and number of species of living organisms, if we divide one infusion amongst several vessels which are immediately closed up again, than if we leave our infusion in contact with an unlimited volume of surrounding air. By this means we expose each portion of the liquid to no other germs than those existing in a state of suspension in the