234 CRITIQUES AND ADDRESSES. [x. 



ago, 1 to be models of accurate experimentation and logical 

 reasoning. He strained air through cotton-wool, and 

 found, as Schroeder and Dusch had done, that it con- 

 tained nothing competent to give rise to the development 

 of life in fluids highly fitted for that purpose. But the 

 important further links in the chain of evidence added 

 by Pasteur are three. In the first place he subjected 

 to microscopic examination the cotton- wool which had 

 served as strainer, and found that sundry bodies clearly 

 recognizable as germs, were among the solid particles 

 strained off. Secondly, he proved that these germs 

 were competent to give rise to living forms by simply 

 sowing them in a solution fitted for their development. 

 And, thirdly, he showed that the incapacity of air 

 strained through cotton-wool to give rise to life, was 

 not due to any occult change effected in the constituents 

 of the air by the wool, by proving that the cotton- 

 wool might be dispensed with altogether, and perfectly 

 free access left between the exterior air and that in the 

 experimental flask. If the neck of the flask is drawn 

 out into a tube and bent downwards ; and if, after the 

 contained fluid has been carefully boiled, the tube is 

 heated sufficiently to destroy any germs which may be 

 present in the air which enters as the fluid cools, the 

 apparatus may be left to itself for any time and no life 

 will appear in the fluid. The reason is plain. Although 

 there is free communication between the atmosphere 

 laden with germs and the germless air in the flask, 

 contact between the two takes place only in the tube ; 

 and as the germs cannot fall upwards, and there, are 

 no currents, they never reach the interior of the flask. 

 But if the tube be broken short off where it proceeds 

 from the flask, and free access be thus given to germs 



l " Lectures to Working Men on the Causes of the Phenomena of Organic 

 Nature/' 1363. 



