DEVELOPMENT AND SCOPE OF BACTERIOLOGY 5 



mediately following 1860, when Pasteur conducted a series of experi- 

 ments which were not only important in incontrovertibly refuting the 

 doctrine of spontaneous generation, but in establishing the principles 

 of scientific investigation which have influenced bacteriological re- 

 search since his time.' 



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 succeeded 

 in showing that similar, sterilized " putrescible " liquids, if left in con- 

 tact with air, would remain uncontaminated provided that the en- 

 trance 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 immediately 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 atmospheric levels, 

 in places in which the air was subject to varying degrees of dust con- 

 tamination, he showed an inverse relationship between the purity of 

 the air and the contamination of his flasks with microorganisms. 



The doctrine of spontaneoiis generation had thus received its final 

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

 plete 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 observe and 

 correctly interpret bacterial spores and to demonstrate their high powers 

 of resistance against heat and other deleterious influences. 



' 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." 



