It is really a small matter that one more ferment should be placed in a list of exceptions to the generality of living beings, for whom there is a rigid law in their vital economy which requires for continued life a continuous respiration, a continuous supply of free oxygen. Why, for instance, has Dr. Brefeld omitted the facts bearing on the life of the vibrios of butyric fermentation? Doubtless he thought we were equally mistaken in these: a few actual experiments would have put him right. These remarks on the criticisms of Dr. Brefeld are also applicable to certain observations of M. Moritz Traube’s, although, as regards the principal object of Dr. Brefeld’s attack, we are indebted to M. Traube for our defence. This gentleman maintained the exactness of our results before the Chemical Society of Berlin, proving by fresh experiments that yeast is able to live and multiply without the intervention of oxygen. “My researches,” he said, “confirm in an indisputable manner M. Pasteur’s assertion that the multiplication of yeast can take place in media which contain no trace of free oxygen.... M. Brefeld’s assertion to the contrary is erroneous.” But, immediately afterwards, M. Traube adds: “Have we here a confirmation of Pasteur’s theory? By no means. The results of my experiments demonstrate, on the contrary, that this theory has no sure foundation.” What were these results? Whilst proving that yeast could live without air, M. Traube, as we ourselves did, found that it had great difficulty in living under these conditions; indeed he never succeeded in obtaining more than the first stages of true fermentation. This was doubtless for the two following reasons—first, in consequence of the accidental production of secondary and diseased fermentations, which frequently prevent the propagation of alcoholic ferment; and, secondly, in consequence of the original exhausted condition of the yeast employed. As long ago as 1861 we pointed out the slowness and difficulty of the vital action of yeast when deprived of air, and a little way back, in the preceding paragraph, we have called attention to certain fermentations that cannot be completed under such conditions without going into the causes of these peculiarities. M. Traube expresses himself thus: “Pasteur’s conclusion, that yeast in the absence of air is able to derive the oxygen necessary for its development from sugar, is erroneous; its increase is arrested, even when the greater part of the sugar still remains undecomposed. _It is in a mixture of albuminous substances that yeast, when deprived of air, finds the materials for its development._” This last assertion of M. Traube’s is entirely disproved by those fermentation experiments in which, after suppressing the presence of albuminous substances, the action, nevertheless, went on in a purely inorganic medium, out of contact with air, a fact of which we shall give irrefutable proofs.[134]