The caseous ferment, however, must not be exposed to heat, under the afore-mentioned conditions, when it is too young. At the commencement of its development, for instance, within a few days of having been sown. In such case, it would be in danger of perishing, probably in consequence of the tenderness of its tissues. At the end of a fermentation, and even several months afterwards, it might be safely heated to 50° C. (122° Fahr.) without any harm to it. “Low” yeast also can withstand a temperature of 50° C. in the medium in question. Footnote 107: Although we believe that the aërobian ferment of a particular yeast may be produced by a kind of transformation of the cells of the latter, yet we admit that this question is open to some doubt. The facts which we unexpectedly discovered in connection with the _caseous_ ferment should make us extremely careful, and disposed to inquire whether aërobian ferments do not originally, in a state of intermixture, form part of the ferments from which they spring. One reason which might incline us to believe this, is the fact that a ferment sometimes perishes without the appearance of aërobian ferment on the surface. There is nothing very natural indeed in the hypothesis that we advance, which sets aside the supposed intermixture; but, on the other hand, if the aërobian ferment is a particular ferment, simply intermixed with some other variety and developed by change of conditions, how are we to account for its great resemblance in appearance and mode of budding to the ferment on the surface of which it appears? This resemblance, however, might be accounted for very naturally if the two ferments were originally related. Footnote 108: We insist on this fact, that Fig. 50 represents the forms on revival of the aërobian ferment of _saccharomyces pastorianus, when this has grown in a mineral medium_. When produced on the surface of fermented wort, the aërobian ferment of which we are speaking presents no peculiarity, nor is there any irregularity in its forms or in its development, and when we proceed to cultivate it in a natural saccharine medium, or in wort, it does not produce any forms of _dematium_, as in the preceding case; but the reason of this is that, in consequence of the nature of the first medium, which is better adapted to its nutrition, it assumes at once, in the second medium, the forms of deposit-yeast in the course of ordinary germination. Footnote 109: “La cassure de la levûre.” Footnote 110: We have reason to believe that the ratio of the proportions of these ferments depends greatly on the climatic conditions preceding the period of vintage, on the state of dryness or humidity, as well as the temperature at the time of gathering the grapes, and also on the nature of the vines. CHAPTER VI. THE PHYSIOLOGICAL THEORY OF FERMENTATION.