By an ingenious method of filtration, which is fully described in his memoir,[15] M. Pasteur separated from the air that passed through his apparatus the solid particles which it contained. This search convinced him that there were, as he says, “constantly in ordinary air a variable number of corpuscles whose form and structure declare them to be organized.” Some of these, he thinks, resemble the spores of fungi, and others the ova of ciliated infusoria, though he adds:--“But as to affirming that this is a spore, much less the spore of any definite species, and that one is an egg, and belonging to such an infusorium, I believe that this is not possible.” He limits himself, in fact, to the statements, that the corpuscles which he found, were (in his opinion) evidently organized; that they resembled in form and appearance the germs of the lower kinds of organisms; and that, from their variety in size, they probably belonged to many different sorts of living things. Even here, therefore, we have to do with the impressions of M. Pasteur, rather than with verified statements. All that has been established by his direct investigation as to the nature of the solid bodies contained in the atmosphere is this: that the air contains a number of round or ovoidal corpuscles, often quite structureless, which he could not distinguish from the spores of fungi[16]--some of which, being about the right size, were round or ovoidal, and structureless. In addition, however, it has been shown that the air contains other rounded corpuscles which are similarly structureless, though composed of silica or starch. It may therefore be asked, in the first place, whether the conclusion is a sufficiently safe one that many of the corpuscles found by M. Pasteur were spores of fungi; and in the next place, supposing this to have been established, whether such spores were living or dead. These questions would have been answered satisfactorily if M. Pasteur could state that he had actually watched the development of such corpuscles, in some suitable apparatus, into distinct organisms. But any such development, he distinctly states, he never witnessed. He says[17]:--“What would have been the better and more direct course would have been to follow the development of these germs with the microscope. Such was my intention; but the apparatus which I had devised for this purpose not having been delivered to me at a convenient time, I was diverted from this investigation by other work.” The evidence which he does adduce, in subsequent portions of his memoir, in order to prove that some of these corpuscles were really “fertile germs,” is almost valueless, because all the facts are open to another interpretation, which is just as much, nay, even more, in accordance with Baron Liebig’s than with his own doctrine of fermentation. [15] ‘Annales de Chimie et de Physique,’ 1862, t. lxiv. p. 24.