20 Introduction 



the time of Leeuwenhoek were living organisms — vegetable forms- 

 capable of growth. . _ 



So long as yeast was looked upon as an inert substance it was 

 impossible to understand how it could impart fermentation to other 

 substances; but when it was shown by Latour that the essential 

 element of yeast was a growing plant, the phenomenon became a 

 perfectly natural consequence of hf e. Not only the alcoholic, but 

 also the acetic, lactic, and butyric fermentations have been shown 

 to result from the energy of low forms of vegetable life, chiefly 

 bacterial in nature. Prejudice, however, prevented many chemists 

 from accepting this view of the subject, and Liebig strenuously 

 adhered to his theory that fermentation was the result of the 

 internal molecular movements which a body in the course of de- 

 composition communicates to other matter whose elements are 

 connected by a very feeble aflS.nity. 



Pasteur was the first to prove that fermentation is an ordinary 

 chemic transformation of certain substances, taking place as the 

 result of the action of living cells, and that the capacity to produce it 

 resides in all animal and vegetable cells, though in var3dng degree. 



In 1862 he pubUshed a paper " On the Organized Corpuscles Exist- 

 ing in the Atmosphere," in which he showed that many of the 

 floating particles collected from the atmosphere of his laboratory 

 were organized bodies. If these were planted in sterile infusions, 

 abundant crops of micro-organisms were obtained. By the "use of 

 more refined methods he repeated the experiments of others, and 

 showed clearly that "the cause which communicated life to his 

 infusions came from the air, but was not evenly distributed 

 through it." , 



Three years later he showed that the organized corpuscles which 

 he had found in the air were the spores or seeds of minute plants, 

 and that many of them possessed tie property of withstanding the 

 temperature of boiling water — a property which explained the 

 peculiar results of many previous experimenters, who failed to 

 prevent the development of life in boiled liquids inclosed in her- 

 metically sealed flasks. 



Chevreul and Pasteur, by having proved that animal solids do not 

 putrefy or decompose if kept free from the access of germs, suggested 

 to surgeons that putrefaction in wounds is due rather to the entrance 

 of something from without than to changes within. The deadly 

 nature of the discharges from putrescent wounds had been shown in 

 a rough manner by Gaspard as early as 1822 by injecting some of the 

 material into the veins of animals. 



m. MEDICAL AND SURGICAL CONTRIBUTIONS; THE STUDY OF THE 

 INFECTIOUS DISEASES 



Probably the first writing in which a direct relationship between 

 micro-organisms and disease is suggested is by Varro, who says: 



