The opposition of the properties of the two tartaric acids, right and left, at the moment when the conditions of life and nutrition of an organised being intervened, showed themselves still more strikingly in a very curious experiment made by Pasteur. He was the first to prove that mildew could live and multiply on a purely mineral soil, composed, for example, of the phosphates of potash, of magnesia, and an ammoniacal salt of an organic acid. For such a development of vegetable life he employed the seed of _penicillium glaucum_, which is to be found everywhere as common mould, and to which he offered, as its only carbon aliment, paratartaric acid. At the end of a little time the left-handed tartaric acid appeared. Now this left-handed acid could only show itself on the condition that a rigorously equal quantity of the right-handed acid had been decomposed. The carbon of the tartaric acid evidently supplied to the little plant the carbon that was necessary for the formation of its constituents and all their organic accessories. If the microscopic seed of penicillium sown upon this soil was not formed of dissymmetric elements, as is the case with all other vegetable substances, its development, its life, its fructification would accommodate themselves equally well with the left-handed tartaric acid as with the right. The fact that the left-handed tartaric acid is less assimilable than its opposite is due solely and evidently to the dissymmetry of one or other of the primordial substances of the little plant. * * * * * Thus for the first time was introduced into physiological studies and considerations the fact of the influence of the molecular dissymmetry of natural organic products. Pasteur always speaks with enthusiasm of the grand future reserved for researches which have this influence for their object; for molecular dissymmetry is the only sharp line of demarcation which exists between the chemistry of inorganic and that of organic nature. FOOTNOTE: [7] [M. Pasteur appears to use the word _devenir_ as a substantive in a sense equivalent to the German _Werdende_.] _FERMENTATION._ Arrived at this unexpected turn in the road which he had hitherto pursued, Pasteur paused for an instant. Should he commit himself to the course which abruptly opened before him? His scientific instincts urged him to do so, but the prudence and reserve which show themselves to be the basis of his character, whenever he finds himself called upon to make a choice of which the necessity is not absolutely demonstrated, held him back. Was it not wiser to continue in the domain of molecular physics and chemistry? M. Biot counselled his doing so; the route had been made plain, success awaited him at each step, but an incident connected with the University triumphed over his hesitations.