162 Professor Tyndall [Jan. 16, 



right and the other to the left. This self-same molecular peculiarity 

 enabled the little fungus to live and thrive upon the one class of 

 molecules, while leaving the other class intact. 



In making these extremely curious observations, Pasteur found 

 himself confronted by the general question of fermentation. In 1837, 

 Cagniard - Latour and Schwann had independently proved the 

 alcoholic ferment to be a budding microscopic vegetable. Spui-red by 

 this discovery and strengthened by his own, Pasteur rapidly closed 

 witb the idea that all ferments were living organisms, and that the 

 substances usually regarded as ferments were in reality the food of 

 the! ferments. Thus the sugar of the wort in the manufacture of beer, 

 the sugar of the grape in the manufacture of wine, the sugar of the 

 cherry in the manufacture of kirsch, the sugar of the apple in the 

 manufacture of cider, and, it might perhaps be added, the sugar of 

 the gooseberry in the manufacture of champagne, is decomposed by 

 the little organism which derives from the sugar the oxygen necessary 

 for its existence. One of the products of this decomposition is our 

 familiar alcohol. 



When I studied at the University of Marburg, one of the luxuries 

 of student life consisted of pancakes and sour milk. Whence this 

 pleasant acidity? It was proved by Pasteur to be due to living 

 microscopic rods — the lactic acid ferment — which grew and multiplied 

 in the milk. The butyric acid ferment was also proved to be an 

 organism. The acidity of sour wines was proved to be due to a 

 minute microscopic plant called Mycoderma aceti. By the action of 

 this plant, wine is transformed into vinegar, the vast industries of 

 Orleans and other places being based upon its operations. Ruinous 

 losses had frequently been incurred by the souring of French wines, 

 but Pasteur proved to the wine-grower that by simply heating his 

 wine to a temperature of 122*^ Fahrenheit — a temperature which in no 

 way alters the quality of the wine— the injurious organisms are all 

 destroyed, the wine being thereby permanently protected. 



The sourness, putridity, and other maladies of beer were traced 

 by Pasteur to special organisms — ferments of disease, as he rightly 

 calls them— which, mingling with the torula or true yeast plant, 

 added their offensive products to the pure alcohol. Vast losses were 

 frequently incurred by the use of bad yeast, where five minutes' 

 examination with the microscope would have revealed to the brewer 

 the cause of the badness, and prevented him from using the yeast. 

 Under the head of fermentation, Pasteur rightly placed the phe- 

 nomena of putrefaction, which he studied with admirable thorough- 

 ness and skill. All the objectionable odours of putrefying flesh 

 result from decompositions set up by microscopic organisms. Keep 

 your meat free from such organisms — kill them by heat or deaden 

 them by cold — and you can have no putrefaction. Schwann, whom I 

 have already mentioned, was the first to upset the doctrine propounded 

 by Gay Lussac, that putrefaction was caused by atmospheric oxygen, 

 and to prove that it was not the air, but living germs suspended in 



