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was the beginning of the series of researches by the aid of which Du Bois-Reymond, Helmholtz, and others have built up the general physiology of the nerves and muscles. It was the first instance, says Du Bois-Reymond, of the examination of an eminently vital force as if it were a physical one, and of the mathematical expression in figures of the laws of its action.

Schwann assisted, with the professors at Berlin, in the preparation of the Encyclopedic Dictionary of the Medical Sciences, to which he contributed the articles on vessels, hematose, urinary secretion, and cutaneous secretion. At this period, also, he began the experiments which led up to the discovery of the digestive ferment, pepsin; and the principles which he set forth on the subject are essentially the same as are still taught, the elucidation of a few details being all that has been added.

In one of the theses attached to his inaugural dissertation, Schwann had opposed the theory of spontaneous generation, which had begun to prevail again, after a general abandonment of Spallanzani's germ theory. The absence of microbes from preparations which had been hermetically sealed was attributed to the deprivation of oxygen. Schwann and Franz Schulze labored independently to disprove this view. Schulze showed that vegetable and animal infusions could be preserved for months in the presence of air and after renewing supplies of air, if the air was first passed over sulphuric acid to kill the germs in it. Schwann communicated to the Society of German Naturalists and Physicians the results of similar experiments, and of others in which he destroyed the germs by calcination. He explained putrefaction as a work of decomposition by the germs developing themselves at the expense of the organic substance, in proof of which he showed that arsenic and corrosive sublimate, which were poisonous to infusoria, were also the best preservatives against putrefaction. It remained to be shown that the calcination of the air did not deprive it of its essential properties of sustaining respiration and promoting alcoholic fermentation—for the advocates of spontaneous generation might say that the development of life was prevented by asphyxiation. Schwann's view was sustained when he found that frogs suffered no inconvenience in calcined air; but, when it came to apply the test to the fermentation of alcohol, no fermentation took place. Schwann was not discouraged by this, but proclaiming a new discovery, that yeast was an organic growth, and working out experiments to prove it, converted the apparently hostile result into an additional support to his theory. These ideas did not receive at once the support they deserved. They had a formidable adversary in Liebig, who set forth another theory of fermentation, and ridiculed them with a parody. Schwann, averse to controversy, made no answer to