12 



have obtained access to the culture fluid ; for the micro- 

 scopic dimensions of the particle prevent continuous ob- 

 servation during the transfer, and the morphological 

 similarity of different varieties, especially among the 

 micrococci, render individual recognition impossible. 

 Every object which can come into contact with the liquid 

 or the particle under examination the skin of the ani- 

 mal from which the tissue is transferred, the instruments, 

 the vessel or slide containing the nutrient material, the 

 material itself, the surrounding air, so far as possible- 

 must be sterilized, liberated from all contained and adhe- 

 rent organisms ; and even then there remains an ele- 

 ment of uncertainty, since it is impossible (by the ordi- 

 nary methods of cultivation) to demonstrate that these 

 precautions have been efficient. These difficulties of 

 sterilization cannot be fully appreciated without actual 

 experience, which soon demonstrates that the greatest 

 care, and attention is often impotent to secure the isola- 

 tion of a given species from other bacteria ; Bastian's 

 famous experiments in support of spontaneous genera- 

 tion may serve as an illustration. 



In his earlier work, though carefully and conscien- 

 tiously performed, the apparently spontaneous appear- 

 ance of bacteria in various animal and vegetable infu- 

 sions was easily explained by his failure to previously 

 heat the glass vessels in which the infusions were kept ; 

 for the observance of this now elementary precaution 

 prevented the appearance of organisms in the liquids. 

 But later experiments seemed indeed unassailable ; he 

 found that thoroughly boiled urine remained, in a pre- 

 viously heated and well-stoppered flask, perfectly free 

 from organisms ; when however the urine was made al- 

 kaline by the addition of a caustic potash solution, also 

 previously boiled, the conditions remaining otherwise 

 unchanged, bacteria were soon developed in immense 

 numbers. Bastian explained by the hypothesis that al- 

 kaline was more favorable than acid urine to the genera- 

 tion of these organisms. Pasteur, skeptical as to the 

 accuracy of Bastian's manipulations, repeated the ex^ 



