When Pasteur offered the same explanation of the origin of vinegar he found a strenuous opponent in Liebig, the great chemist. Liebig admitted the existence of specific substances, {305} called ferments, but said that they were nitrogenous compounds in unstable equilibrium as regards their composition, and with a marked tendency to undergo alteration when exposed to the air or free oxygen. These alterations, once begun, affect also the liquids in which the ferments are contained--milk, blood, sugar solutions and the like. Theodore Schwann had shown the existence of certain yeast-like bodies in fermenting liquids, but these were considered to be effects, not causes, of the fermentation, and even Schwann, himself, believed that they originated in the liquids in which they were found. It remained for Pasteur to demonstrate, as he did by a brilliant series of ingenious and conclusive experiments, that ferments are living cells, that they never originate except from previous cells of the same species, and that no fermentation takes place unless they are present. The changes that take place in organic liquids when exposed to the air and the frequent development in such liquids of moving bodies evidently possessed of life constituted, before Pasteur's time, the principal reason for believing that life might originate from some special combination of chemical forces, and without the necessity for preceding life of the same species as its efficient cause. The new explanation of fermentation greatly weakened the position of those who believed in spontaneous generation--that is, the origin of life from dead matter under certain specially favorable circumstances. Pasteur proceeded to show, by rigid demonstration, that if all life were destroyed in organic substances, living beings never originated in them unless living seeds from the air gained access to them. After a meat solution has been thoroughly boiled nothing living develops in it, even though the air is allowed free access, if the air admitted has been previously filtered through cotton, He showed that even the {306} bending of the neck of the tube into the shape of an "S," so as to prevent the entrance of dust particles, suffices to protect the most changeable organic material from the growth of micro-organisms in it. His teaching was not accepted at once. Details of his experiments were impugned. Apparently complete counter-demonstrations were made, but Pasteur knew how, by his marvellous intuition, to detect the fallacy of supposed demonstration, and to invent new crucial tests of the proof of biological succession.