As regards the explanation of the influence which acidity or alkalinity exerts in diminishing or increasing the temperature required to protect infusions and organic matters from ultimate change, although this is a subject which claims special study, we are inclined to believe that acidity permits, and alkalinity prevents, the penetration of moisture into the interior of the cells of the germs belonging to infusions, so that in heating the outer cases of these cells in an alkaline medium we heat the germs in a dry state; and in heating the outer cases in an acid medium we heat the germs when they are moist. We all know that these conditions make a great difference in the resistance which bodies offer to the action of heat. A particle of mould which, in a moist state, cannot survive a temperature of from 60° C. to 100° C. (140° to 212° F.), will preserve its fecundity even if heated to 120° C. (248° F.), if it has been previously well dried.[23] The nature of the spontaneous productions which we see appear in organic liquids is affected to a remarkable extent by the smallest change in the compositions of those liquids. Generally speaking, as we have often proved in our former works, a feeble acidity is unfavourable to the development of bacteria and infusoria, and, on the other hand, favourable to the growth of mould. A liquid which is neutral or of feeble alkalinity behaves in an exactly inverse manner. Those who support the theory of spontaneous generation seek to find in the natural differences between the organic productions of various liquids which are simultaneously exposed to the same atmosphere, an argument in favour of their doctrine. These differences, however, are only an effect of the greater or less fitness of a peculiar liquid for a certain kind of growth. When an acid organic liquid, such as the must of grapes for example, is exposed freely to the air, it is besieged simultaneously by spores of _mucedines_, as well as germs of _bacteria_, _leptothrix_, _vibrios_, _&c._, but the latter germs are checked in their development, if, indeed, they are not actually destroyed, by the acidity of the liquid; and we should never find them there in their adult state. Most moulds, on the other hand, thrive in acid liquids, and therefore they are generally found alone. If we began by saturating must with carbonate of lime, previously dried or otherwise, we should observe opposite phenomena; bacteria, lactic ferments and butyric vibrios would invade our fields long before the spores of mould had time to grow, since the germination of these proceeds very languidly in neutral or alkaline liquids, and an infusion once occupied by a living organism has much trouble in nourishing others; the early developments consume the alimentary substances, especially the oxygen.