Now, the most remarkable fact among the phenomena of fermentation, and that which must chiefly be kept in view in the explanation of the process, is this, that the ready-formed cells, after being washed, effect the conversion of pure cane-sugar into grape-sugar, and its resolution into a volume of vapour and alcohol, and that the elements of the sugar are obtained without any loss in these new forms; that consequently, since three pounds of yeast, considered in the dry state, decompose two hundred-weight of sugar, a very powerful action takes place, without any notable consumption of matter for the vital purpose of forming cells. If the property of exciting fermentation depended on the development, propagation, and increase of yeast-cells, these cells would be incapable of causing fermentation in pure solutions of sugar, in which the other conditions necessary for the manifestation of the vital properties, and especially the nitrogenous matters necessary for the production of the contents of the cells, are absent. Experiment has proved that in this case the yeast-cells cause fermentation, not because they propagate their kind, but in consequence of the decomposition of their nitrogenous contents, which are resolved into ammonia and other products--that is, in consequence of a decomposition which is exactly the opposite of an organic formative process. The yeast, when brought into contact successively with the new portions of sugar, loses by degrees entirely its power of causing fermentation, and at last nothing is left in the liquid but its non-nitrogenous envelopes or cell-walls.[28] [28] Liebig: Letters on Chemistry. On the other hand, it may be admitted that fungi and agarics, and all that lives, vegetable and animal, contaminate the air when dead; they absorb oxygen and give out vapours of which some are clearly detrimental to human life. The effect of breathing air so contaminated is in some countries immediate--that is, the incubation of the poison requires only a few days, in others many months. Waters in a state of fermentation or putrefaction seem to poison the plants themselves, for duckweed and other swimming plants die, and the swallow and the marten disappear. On the wide ocean and over the absolute desert, the air is always pure, nothing living is decomposing; but watch the mud coasts, and observe the pestilential effects of sea water when suffered to evaporate, or still more when confined to a locality and suffered to decompose. In the ancient world, as in the modern, nature teemed with life, since a cubic inch of the fossil infusoria, contains 41,000 millions of individuals. The microscopic shell fish called entomostraca were equally abundant.