PRODUCED BY ALCOHOLIC FERMENTS. 163 



and if care were taken that the air with which they sub- 

 sequently came into contact, was freed from its germs. Both 

 these experimenters, therefore, fitted their flasks with doubly 

 bored stoppers through which bent glass tubes passed. The 

 tubes served for the passage of the air into the flasks. In 

 Schulze's experiments, the air which was drawn into the 

 flask after boiling was purified by passing through sulphuric 

 acid. Schwann, on the other hand, effected this purification 

 by heat. The experiments of both showed that any quantity 

 of air, however large, could be passed through such decoctions 

 without putrefaction, fermentation or the development of 

 micro-organisms taking place ; these experiments prove, 

 therefore, exactly the opposite of Needham's statement, 

 and they confirm the correctness of Spallanzani's results. 



The years 1836-1839 are notable in the history of micro- 

 biology for the epoch-making researches of Cagniard Latour 

 and Theodor Schwann, who, for the first time, proved that 

 yeast consists of living cells, and that it is these which bring 

 about alcoholic fermentation. What had been previously 

 stated in connection with this, consisted only of conjecture. 



Kiitzing simultaneously arrived at a similar result, and in 

 his treatise he not only gives descriptions and figures repre- 

 senting yeast cells, but also the acetic ferment and various 

 mould fungi.* .-He distinctly noted the difference between 

 yeast cells and the cells of the acetic ferment; and faulty 

 as his figures of the latter are, they still show that even at 

 that time he had really discovered one of the species of 

 bacteria which bring about the production of acetic acid. 

 This was described by him under the systematic name 

 Ulvina aceti on account of its giving the acid fermentation. 

 When wine and beer become sour from the formation of 

 acetic acid, the effect is spoken of as a disease, and is indeed 

 regarded as one of the very worst. In Kiitzing's writings 



* Kiitzing, " Mikroscopische Untersuchungen iiber die Hefe und Essigmutter," 

 'Journal f. praktische Chem.,' 1837, 2 Bd. p. 385. 



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