Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/105

 FERMENTATION 95 sist of minute globular or ovoid particles. Microscopes in his time were very imperfect or he would have made a great discovery. Schwann and Cagniard-Latour, who (about 1838, and independently of each other) resumed the old Dutchman s inquiry, used the better instruments of their time, and discovered that Leuwenhock s globules are membranous bags, which exhibit all the morphologic char acteristics of vegetable cells, and, like these, when brought under the proper conditions, increase and multiply in the biologic sense. Taking this together with the long known fact that in vinous fermentation the yeast increases as the process progresses, they naturally concluded that yeast is a species of plant, and that it is the life of that plant which somehow or other causes the chemical change. It is the special merit of Schwann to have adduced powerful experi mental evidence in favour of this view. In his case, the observations on yeast were incidental only to a more com prehensive investigation, the original aim of which had been to solve the great question of spontaneous generation. Pro cesses of putrefaction having long been known to be invari ably accompanied by the formation of vibriones and other microscopic organisms endowed with voluntary motion, he prepared infusions of flesh and other putrescible matters in glass flasks, and, after having hermetically closed these, ex posed them for a time to the heat of boiling water, so as to destroy every trace of living germs that might be present. The contents, when preserved in that condition for ever so long, showed no sign of putrefaction or of life of any kind. But when exposed to the air they did putrefy, and soon swarmed with living organisms of various kinds. Obvi ously it was the air which caused this two-fold change. But then the air which had been shut up with the infusions did not act. This, however, might have been owing to an absorption of the oxygen by the juices. Schwann therefore, in another set of experiments, allowed the boiled (and con sequently germless) infusions to communicate freely with the atmosphere, in such a manner, however, that no particle of air could enter the flasks without having first passed through a red-hot glass tube, and thus been freed from any germs that might float about in it. In this case the air had fair play in a chemical sense, but yet, not only did no life of any kind make its appearance, but even the chemical changes failed to set in. Exactly similar results were ob tained by Schwann in experiments with grape juice, whether previously mixed or not with yeast. Gay-Lussac s famous experiment failed when the air-bell, before being admitted to the juice, had been heated, and thus freed from living germs. In a few of these experiments, it is true, the re sults were contradictory to the general evidence afforded by the rest of the work. But Schwann had no doubt in his mind about the close analogy between vinous fermentation and putrefaction ; and as the putrefaction experiments had all given one and the same answer, he explained these ano malies as having been caused by unobserved slips in the respective experiments, and did not admit them to invali date his general proposition that both putrefaction and fer mentation are inseparably connected with characteristic biologic phenomena; the less so, as his experiments on the action of certain antiseptics had shown that what is an &quot; antiseptic &quot; to a fermentative change is a poison to the organisms characteristic of the case. Thus, for instance, lie found that white arsenic 1 and corrosive sublimate, being poisonous to both plants and animals, stop both putrefac tion and fermentation ; while extract of nux vomica, being destructive of animal but not of vegetable life, prevents putrefaction, but does not interfere with vinous fermenta tion. The mechanism of the latter process he imagined to 1 According to Schutzenberger, who, however, does not quote his authority, arsenious acid does not impede vinous fermentation. consist probably in this, that the &quot;sugar-fungus&quot; (the yeast) lives at the expense of the nitrogenous matters and of the sugar of the juice, and that those of the elements of these substances which the plant does not assimilate are elimi nated chiefly as alcohol. This special theory of Schwann s, as the reader is aware, is not quite correct, but it does not affect his general views on the phenomenon, which were fully confirmed by subsequent investigators. Amongst these we may mention Helmholtz, who showed that oxygen evolved by electrolysis from water does not, like air, induce vinous fermentation. The same observer showed that boiled grape juice, when tied up in a bladder, does not ferment, even when suspended within a tub full of fermenting juice. The evidence afforded by this experiment was considerably strengthened by Mitscherlich, who proved that even a septum of filter paper effectually stops the propagation of the reac tion. More striking still is an experiment which was made, many years later, by H. Hoffmann. He took a test-tube full of sugar water, and by a plug of cotton wool inserted within it divided the liquid into two parts. To the upper part he added yeast, which of course induced fermentation there ; but the change did not propagate itself through the cotton wool to the lower portion. The same material had done good service some years before in the hands of Schroder and Dusch, who proved in 1854, by a most extensive series of experiments, that the something in air which enables it to start fermentative changes in boiled infusions of meat or malt, in grape juice, &amp;lt;fec., can be effectually removed by fil tration of the air through cotton wool. It is true the &quot;&c.&quot; here does not include milk, which they found to turn sour in filtered as well as in ordinary air, but this exception was subsequently explained away by Pasteur, who found that germs immersed in alkaline liquids may survive tempera tures considerably higher than 100 C. A number of other important researches, which led to substantially similar results, must be passed over here, and may be, because what we have quoted has never been dis proved, and is consequently quite sufficient to show that, in the case of vinous fermentation and putrefaction at any rate, those atomic motions, which, according to Liebig, cause the disintegration of the fermenting substances, if the notion is to be maintained at all, cannot be admitted to have an existence outside the living bodies of certain organisms characteristic of the respective changes. To any unpre judiced person this would appear to be sound logic ; but Liebig did not see it, and for a long time he had the majority of chemists at least on his side. No reasonable person could have denied the irresistible force of the argu ments of Schwann and his followers ; but these chemists somehow or other managed to ignore the facts, until Pasteur, by means of a most thorough and extensive experi mental research (of which the principal portions were published from 1857 to 1861), simply forced the attention of everybody to the physiological side of the subject, and, by absolutely unimpeachable evidence, proved that Schwann s views are substantially correct. Of this investigation it is impossible to speak otherwise than in terms of the highest admiration. Even the purely critical portion of Pasteur s work would be enough to immortalize his name* He did the whole of the work of Schwann and the rest of his pre decessors over again, modifying and perfecting the experi mental methods, so as to silence any objection or doubt that might possibly be raised, repeating and multiplying his experiments until every proposition was firmly esta blished. But his work was synthetical as well as analyti cal. Some of his discoveries will be noticed below ; suffice it here to mention one of the general results which he arrived at. Vinous fermentation is only one of a number of fermen tative changes to which sugar is liable. The same substance sugar, which, when placed under certain conditions, breaks