[Chap. XLII THE BIOLOGY OF BACTERIA 527 



Sterilization by heat and chemicals. In 1776 Spallanzani ( 1729-1799 ) , 

 an Italian, proved that if meat broth is heated for an hour or two in 

 hermetically sealed flasks, no microorganisms develop in the broth. The 

 methods employed in this simple experiment were used by Appert in 

 1810 to exclude bacteria and molds from foodstuffs, and are the basis 

 of our canning industry. The objection was immediately made, in the 

 case of Spallanzani's experiment, that the heating had spoiled the air 

 inside the flask so that no organism could live there, and therefore that 

 spontaneous generation had not been disproved. Upon exposure to air 

 the broth was soon teeming with organisms. For this objection Spal- 

 lanzani's experiments provided no explanation. 



Schultze in 1836 obtained similar results, although he admitted air 

 to his previously heated flasks of broth through tubes containing strong 

 acids and other chemical compounds. Schwann (1810-1882) the same 

 year allowed the air to enter the flasks through intensely heated tubes. 

 It was argued by opponents that both the heat and the chemicals had 

 altered the air. Von Schroeder and von Dusch in 1854 filtered air en- 

 tering the flasks through cotton or wool plugs, and no organisms de^'el- 

 oped in the flasks of broth. These experiments are the forerunners of 

 the modern use of cotton plugs in culture flasks in all pathology and 

 bacteriology laboratories. 



The chemical theory of fermentation. Owing to poor techniques, other 

 investigators often failed to confirm the results of these experiments or 

 to secure consistent results, and the belief in spontaneous generation 

 continued. About this time the so-called chemical theory of fermenta- 

 tion was current, and this indirectly gave comfort to the proponents of 

 spontaneous generation. This theory, supported by Liebig ( 1803-1873 ) 

 and others, assumed that in decay, or in fermentation, large molecules 

 simph disintegrated into smaller ones, and that fresh meat spoiled be- 

 cause it came into contact with other spoiled meat which initiated the 

 breaking up of the molecules. In other words, this theory would imply 

 that microorganisms, though thev may accompany decay and fermenta- 

 tion, are not the causes of these putrefacti\ e processes. 



Louis Pasteur. A few people had suggested as early as the 18th cen- 

 turv that bacteria, or certain microscopic agents, are the causes of fer- 

 mentation and disease. But it was not until the latter half of the 19th 

 centurv that Louis Pasteur (1822-1895) with a series of brilliantly 

 planned and expertly conducted experiments finally' eliminated the idea 

 of spontaneous generation as a serious explanation for these phenomena. 



