If, therefore, M. Pasteur still maintains the truth of his theory, it should be distinctly understood that it rests originally, not upon established facts, but upon a mere hypothesis--the hypothesis that the air teems with multitudes of invisible _Bacteria_ germs. He is driven to such a doctrine, not only by his own confessions concerning _Bacteria_, but also by the microscopical evidence to which I have referred. So that in explaining the results of any experiments made with the view of throwing light upon the _cause_ of fermentation or putrefaction, it is especially necessary to bear in mind two considerations:-- I. That dust filtered from the atmosphere cannot be proved to include living _Bacteria_; though it is known to contain a multitude of organic particles which may be capable in the presence of water, in accordance with Liebig’s hypothesis, of acting as ferments. II. It must also be recollected that, in the opinion of many, Life represents a higher function which is displayed by certain kinds of organic matter; and that this higher function may be deteriorated or rendered non-existent by an amount of heat which might not be adequate to decompose the organic matter itself. It is all the more necessary to call attention to these two considerations, because M. Pasteur invariably speaks as though it had been established that the air contains multitudes of living _Bacteria_, when, really, he had only proved that the air contains a number of corpuscles resembling spores of fungi, &c. And, as I have already intimated, the existence of spores of fungi in the atmosphere, however well established, is of little or no importance as an explanation of the cause of a very large number of fermentations. Their presence is even of still less importance, owing to the fact of the co-existence with these fungus-spores, of multitudes of organic fragments, which--in accordance with the views of Liebig, Gerhardt, and other chemists--are capable of acting as ferments. To this latter consideration M. Pasteur never even alludes when he speaks (_loc. cit._ p. 40) of his “ensemencements,” and of other experiments which are equally, or even more, capable of being interpreted in accordance with Liebig’s views than with his own. Bearing these considerations in mind, we shall be in a better position to enquire into the real interpretation that may be given to many of M. Pasteur’s results, and into the question as to how far the facts which he records are favourable to his own, or to the adverse doctrine concerning the causes of fermentation. * * * * * In the memoir so often alluded to on “The Organized Corpuscles which exist in the Atmosphere,” M. Pasteur adduced various kinds of evidence, tending, as he thought, to show that the first _Bacteria_ which make their appearance in putrefying or fermenting solutions, have been derived from living _Bacteria_ or their “germs,” which pre-existed in the atmosphere.