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question, more especially as regards the part played by oxygen in 

 the process. They showed that a fluid did not undergo putrefaction 

 if the air admitted to it was passed through bulbs containing 

 potash and sulphuric acid, or heated. 



Next Schwann took up the question of vinous fermentation, and, 

 on investigating yeast with the microscope, rediscovered the yeast 

 plant simultaneously with Cagniard Latour. Leeuwenhoek had 

 already seen yeast plants, but had mistaken them for crystals. 

 Schwann devised a simple yet striking experiment to show the direct 

 action of the yeast on a solution of sugar by splitting it up into 

 carbonic acid and alcohol. In a long tube he placed a solution of 

 sugar faintly tinged blue with litmus, and to this he added very little 

 yeast, so that the cells had time to subside. The red colouration 

 of the litmus due to the liberation of the carbonic acid always began 

 to appear at the bottom of the tube. 



In his inaugural dissertation he showed the necessity of air for 

 the development of the chick in ovo, and in his disputation we find 

 " Infusoria non oriuntur generatione pequivoca," showing that his 

 mind had been directed to this subject. 



His researches on artificial gastric digestion, an extension of 

 those of Spallanzani, and Eberle of Wiirzburg (1834), and Purkinje 

 (1837), made it evident that the digestive principle was not to be 

 sought for in the mucus, but in some other as yet unknown body — 

 a body which he called pepsin. He had discovered an impure form 

 of one member of the inorganic ferments now known as enzymes, 

 to use the word coined by W. KUHNE (1837-1900). We have 

 purposely omitted references to biliary and gastric fistukc, to 

 Beaumont's work, derived from a study of the case of Alexis 

 St. Martin, and much else bearing on this subject. 



As to the nature of this body, Midler and Schwann regarded it 

 as a ferment. At this time Mitscherlich explained fermentation as 

 due to " contact." Schwann showed that acidity was necessary to 

 the action of pepsin, and in his researches compared fermentation 

 and digestion, distinguishing them from putrefaction. 



Here we come across LIEBIG (1803-1873) as an opponent of 

 Schwann's doctrine ; for, according to Liebig, fermentation was not 

 due to the presence of lowly organisms. 



" But the great merit of Schwann rests on his ' cell-theory,' enunciated in 1S39, 

 whereby he brought the formation of all tissues — vegetable and animal alike — under one 

 common law. To be sure, Schleiden, in 1838 had recognised and described the process 

 of development in the cells of plants. But Schwann saw clearly the unity of the 

 vegetable and animal processes, and formulated his 'theoria,' 'that there is one 

 universal principle of development for the elementary parts of organisms, however 

 different, and that this principle is the formation of cells.' It is stated that Schwann 

 submitted the MS. of his work l>efore publication, to the Bishop of Maline. 



