Let us not, then, be surprised at the seemingly discordant results arrived at, and at the contradictory observations which have been made in the best faith possible, and with every regard to truth in science. The circumstances which seemed to be identical are merely analogous, but in point of fact are essentially distinct, as proved by the results. Changes inappreciable by human sense and as yet by philosophical instruments, may and no doubt do effect results, to man seemingly contradictory, simply because he comprehends them not. As chemical science makes progress, these differences are being reconciled and understood. Thus, as mere temperature exercises a truly remarkable influence over the nature of the products of fermentation, may it not be the efficient cause of the difference we observe between the malaria of the delta of the Mississippi and that floating near the muddy banks of the Scheldt? The juice of carrots, beet-root, or onions, which is rich in sugar, when allowed to ferment at ordinary temperature yields the same products as grape-sugar, but at a higher temperature the whole decomposition is changed—there is a much less evolution of gas, and no alcohol is formed.
In the fermented liquor there is no longer any sugar, and thus may it be in the great laboratory of nature; the product of the fermentation will assume in one locality a character it does not possess in another. The elements are the same; there is merely a change in temperature.
Are there facts to prove that certain states of transformation or putrefaction in a substance, are likewise propagated to parts or constituents of the living animal body? Such facts exist. On no other principle but that of assimilation can we explain the phenomena of poisoning by the puncture of the living hand in dissecting-rooms, the instrument being impregnated with a fermentescible and putrefactive substance, there undergoing a decomposition. Similar, unquestionably, must be the action of animal poisons, such as that of poisonous substances, whether animal or vegetable, of the poisons giving rise to zymotic diseases, &c.; and such may be the origin of the fevers caused by the unknown principle which must still be connected with the decomposition of organic bodies most frequently found in marshy countries. But before entering more fully on this important matter, I shall first weigh the evidence for and against a theory long fashionable, and which may even now have its supporters—namely, whether fermentation or the revolution of higher or more complex organic vegetable into less complex compounds, be the effect of the vital manifestations of vegetable matters, and whether putrefaction or the same change in animal substances be determined by the development or the presence of animal beings. They who maintain this theory, assume as a natural consequence of the views that the origin of miasmatic or contagious diseases, in so far as they may be referred to the presence of putrefactive processes, must be ascribed to the same or to similar causes.
§ 2. The refutation of this view by Liebig seems satisfactory, and has not yet been satisfactorily replied to. The subject is one of much interest; the theory has furnished a foundation for some unquestionably entirely fallacious ideas concerning the essence of the vital processes generally, of many pathological conditions, and the causes of certain diseases.
These persons regard fermentation, or the resolution of higher or more complex organic vegetable atoms into less complex compounds, as the effect of the vital manifestations of vegetable matters; and putrefaction, or the same change in animal substances, as being determined by the development or the presence of animal beings. They assume as a natural consequence of this view, that the origin of miasmatic or contagious diseases, in so far as referrible to the presence of putrefactive processes, must be ascribed to the same or similar causes.
The most obvious and important considerations in support of this view of fermentation, are derived from observations made on the alcoholic fermentation, and on the yeast of beer and of wine. The microscopic researches of physiologists and botanists have demonstrated that beer or wine yeast consists of single globules strung together, which possess all the properties of living vegetable cells, and resemble very closely certain of the lower family of plants, such as some fungi and algæ.
In fermenting vegetable juices, we observe, after a few days, small points, which grow from within outwards; and these have a granular nucleus, surrounded by a transparent envelope. The simultaneous appearance of the yeast-cells and of the products of decomposition of the sugar, is the chief argument in support of the opinion that the fermentation of sugar is an effect caused by the vital process, a result of the development, growth, and propagation of these low vegetable structures. But if the development increase, and propagation of these vegetable cells or tissues be the cause of fermentation, then in every case where we observe this effect we must suppose that the causes or conditions—namely, sugar, from which the cell-walls are produced, and gluten, which yields their contents—are both present.
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]
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.