volume of air introduced into the vessels; and it so frequently happens that we obtain a variety of germs which, coming into contact with a liquid adapted to their nutrition, without having mixed with others, finally multiply there, in consequence of no other organisms of greater activity occurring to impede their slow and laborious propagation. The nature of the products resulting from raw infusions—that is, those obtained, without heating, from the maceration of organic substances, such as leaves, fruits, grains, or the organs of plants or animals—is much more varied. The reason of this is that such substances generally carry with them not only the particles of dust existing in air that is in motion, but also microscopic parasites, which find a congenial resting-place on their surface or its vicinity. We may cite a few accurate observations on this point, for the subject is one of great interest. If we boil an infusion of hay, and then expose it to contact with the air in a room, all its productions will be derived from such germs as a comparatively still air can carry about; thus, we shall very rarely find any _colpoda_ in our infusion, for the germs of these infusoria, consisting of rather large cells, can scarcely exist in a state of suspension in motionless air, in spite of their extraordinary diffusion in nature. On the other hand, we almost invariably find _colpoda_ in macerations of raw hay. This difference is easily accounted for; the particles of dust adhering to the surface of hay, especially that which comes from marshy districts, contain the germ-cells of _colpoda_ in abundance. The reason is obvious; rain falls on a meadow and forms little pools of water about the roots of the herbs that grow there; this water remains for some time, and very soon swarms with a multitude of infusoria, especially _colpoda_. Dryness follows; the _colpoda_ becomes encysted, and forms a dust that is wafted by the winds on to the blades of grass, so that the mower will carry away not only his hay, but also myriads of _colpoda_, as well as spores of _mucedines_ and other organisms.[25] The maceration of pepper will give us some infusoria hardly ever found in other infusions, the reason of this being that such infusoria exist where the pepper grows—in other words, their germs are exotic. That the infusion of a special plant should give us special infusoria, is scarcely more surprising than the discovery of a particular parasite or insect existing on a particular plant, and not on others of a different species that grow near it. Thus it happens that the ferment-germs of must exist on the surface of grapes, whether detached or in clusters. It is only natural that we should find the organ or the plant that is destined later on to become the food of a parasite serving as a habitation for the germs of that parasite.