6 INTRODUCTION 



results of Needham and Spallanzani and concerning the relation 

 which the subsequent exclusion of the air might bear to the ab- 

 sence of life in the flasks, .the method of heating was applied to 

 the preservation of vinegar by Scheele (1782) and to the preser- 

 vation of foods in general by Appert (1811). The method was 

 quickly introduced into other countries, and developed by va- 

 rious tradesmen, who attempted with more or less success to 

 keep their processes secret. Success in preservation by canning 

 remained somewhat uncertain, as a precise understanding of the 

 underlying scientific principles was still lacking. Schulze (1836) 

 showed that air might be admitted to flasks prepared by Spallan- 

 zani's method, without the development of life and without 

 putrefaction, provided the air were first passed through a series 

 of bulbs containing concentrated sulphuric acid. The subse- 

 quent work of Schroder and van Dusch (1853), who obtained 

 similar success by filtering the air through cotton, of Pasteur and 

 Tyndall (1860-62) who were able to preserve putrescible fluids 

 directly in contact with air, provided the air were rendered per- 

 fectly free from dust, has established the fact that the decom- 

 position ordinarily taking place after exposure to the air is due 

 to the introduction of living germs into the previously sterile 

 material. 



The idea that specific kinds of fermentation are caused by 

 specific kinds of microbes was first clearly put forward by Schwann 

 and Cagniard-Latour (1837), who showed that yeast-cells were 

 living organisms and claimed that the alcoholic fermentation of 

 sugar solutions was due to their growth. The importance of this 

 relationship received little recognition until Pasteur (1860-72), 

 during his extensive and careful researches into the nature of 

 fermentation and the causation of undesirable fermentation (dis- 

 eases of wines and beers), demonstrated conclusively that the 

 kind of decomposition of a fermentable substance depended upon 

 the nature of the substance, the kind of microbes present and the 

 environmental conditions, such as temperature and presence or 

 exclusion of air. The mere introduction of a small number of 



