Liebig, however, does not dwell upon these considerations, which he merely notices in passing, because he is well aware that, as far as the defence of his theory is concerned, they would be mere evasions.  If he had insisted on them, or based his opposition solely upon them, our answer would have been simply this:  “If you do not admit with us that fermentation is correlated with the life and nutrition of the ferment, we agree upon the principal point.  So agreeing, let us examine, if you will, the actual cause of fermentation;—­this is a second question, quite distinct from the first.  Science is built up of successive solutions given to questions of ever increasing subtlety, approaching nearer and nearer towards the very essence of phenomena.  If we proceed to discuss together the question of how living, organized beings act in decomposing fermentable substances, we will be found to fall out once more on your hypothesis of communicated motion, since according to our ideas, the actual cause of fermentation is to be sought, in most cases, in the fact of life without air, which is the characteristic of many ferments.”

Let us briefly see what Liebig thinks of the experiment in which fermentation is produced by the impregnation of a saccharine mineral medium, a result so greatly at variance with his mode of viewing the question. [Footnote:  See our Memoir of 1860 (Annales de Chimie et de Physique, vol. lviii, p. 61, and following, especially pp. 69 and 70, where the details of the experiment will be found).] After deep consideration he pronounces this experiment to be inexact, and the result ill-founded.  Liebig, however, was not one to reject a fact without grave reasons for doing so, or with the sole object of evading a troublesome discussion.  “I have repeated this experiment,” he says, “a great number of times, with the greatest possible care, and have obtained the same results as M. Pasteur, excepting as regards the formation and increase of the ferment.”  It was, however, the formation and increase of the ferment that constituted the point of the experiment.  Our discussion was, therefore, distinctly limited to this:  Liebig denied that the ferment was capable of development in a saccharine mineral medium, whilst we asserted that this development did actually take place, and was comparatively easy to prove.  In 1871 we replied to M. Liebig before the Paris Academy of Sciences in a Note, in which we offered to prepare in a mineral medium, in the presence of a commission to be chosen for the purpose, as great a weight of ferment as Liebig could reasonably demand. [Footnote:  Pasteur, Comptes rendus de l’Academie des Sciences, vol. lxxiii., p. 1419. 1871.] We were bolder than we should, perhaps, have been in 1860; the reason was that our knowledge of the subject had been strengthened by ten years of renewed research.  Liebig did not accept our proposal, nor did he even reply to our Note.  Up to the time of his death, which took place on April