DEVELOPMENT AND SCOPE OF BACTERIOLOGY 5 



this objection, but had also failed to convince. The question was not 

 definitely settled until the years immediately following 1860, when 

 Pasteur conducted a series of experiments which were not only im- 

 portant in incontrovertibly refuting the doctrine of spontaneous 

 generation, but in establishing the principles of scientific investiga- 

 tion which have influenced bacteriological research since his time. 3 



Pasteur attacked the problem from two points of view. In the 

 first place he demonstrated that when air was filtered through cotton- 

 wool innumerable microorganisms were deposited upon the filter. A 

 single shred of such a contaminated filter dropped into a flask of pre- 

 viously sterilized nutritive fluid sufficed to bring about a rapid and 

 luxuriant growth of microorganisms. In the second place, he suc- 

 ceeded in showing that similar, sterilized ' ' putrescible " liquids, if 

 left in contact with air, would remain uncontaminated provided that 

 the entrance of dust particles were prohibited. This he succeeded in 

 doing by devising flasks, the necks of which had been drawn out into 

 fine tubes bent in the form of a U. The ends of these U-tubes, being 

 left open, permitted the sedimentation of dust from the air as far as 

 the lowest angle of the tube, but, in the absence of an air current, 

 no dust was carried up the second arm into the liquid. In such flasks, 

 he showed that no contamination took place but could be immedi- 

 ately induced by slanting the entire apparatus until the liquid was 

 allowed to run into the bent arm of the U-tube. Finally, by exposing 

 a series of flasks containing sterile yeast infusion, at different atmos- 

 pheric levels, in places in which the air was subject to varying 

 degrees of dust contamination, he showed an inverse relationship 

 between the purity of the air and the contamination of his flasks 

 with microorganisms. 



The doctrine of spontaneous generation had thus received its 

 final refutation, except in one particular. It was not yet clear why 

 complete sterility was not always obtained by the application of 

 definite degrees of heat. This final link in the chain of evidence was 

 supplied, some ten years later, by Cohn, who, in 1871, was the first to 



3 In a letter to his foremost opponent, at this period, Pasteur writes: "In 

 experimental science, it is always a mistake not to doubt when facts do not compel 

 affirmation. ' ' 



The critical spirit pervading the scientific thought of that time in France is 

 also well expressed by Oliver Wendell Holmes, who said that he had learned three 

 things in Paris: "Not to take authority when I can have facts, not to guess when 

 I can know, and not to think that a man must take physic because he is sick. ' ' 



