PASTEUR'S STUDY OF MICROBES 137 



which was afterwards called fixed air, or carbonic acid. 

 Leeuwenhoek about 1680 examined yeast by his micro- 

 scopes, and discovered that it is made up of globules 

 which often cohere, and that these globules give off 

 bubbles of gas. Then comes a long interval, during 

 which nothing was done to elucidate the process of 

 fermentation. It was not till 1837 that Caignard- 

 Latour and Schwann, independently of each other, 

 showed that yeast-globules multiply by budding, and 

 are therefore to be set down as living things, probably 

 plants of a simple kind. Twenty years more passed 

 without sensible progress ; during this time chemists 

 were striving to prove that the alcohol was produced 

 by contact-action, and that the globules were of no 

 practical importance. By the year i860 Pasteur was 

 engaged upon the problem. It is well known that he 

 arrived at a firm conviction that living yeast-cells are 

 essential to the production of alcohol. It has since 

 been discovered that the enzyme (unorganised ferment 

 of older writers) secreted by living yeast-cells can 

 change sugar into alcohol after the cells themselves 

 have been destroyed, and that other plants besides 

 yeast-cells secrete the same enzyme when deprived of 

 oxygen. 



Bacteria. — Another and even more important chapter 

 in the history of air-wafted organisms was opened by 

 the indefatigable Leeuwenhoek. In 1683 he wrote a 

 letter to the Royal Society which makes mention for 

 the first time of bacteria, which he found upon his own 

 teeth, and described as minute rods ; some of them 

 moved with surprising agility. For nearly two hundred 

 years little more was done. A few bacteria were named 

 and classified, and there the matter rested until Schwann 

 proved experimentally that putrefaction is just as much 



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