18 MICRO-ORGANISMS AND FERMENTATION. 



condition that they are proof to every contamination from 

 without. Pasteur's flasks satisfy this demand in the highest 

 degree. 1 The illustration shows this flask in the slightly 

 modified form employed in the Carlsberg Physiological 

 Laboratory directed by Hansen. When the nutritive liquid is 

 boiled, the steam first escapes through the wide straight tube, 

 at the end of which is a piece of India-rubber tubing ; when 

 this is closed the only outlet for the steam is through the 

 bent tube. After some time the flask is taken from the 

 sand-bath, and the bent tube may be closed with a plug of 

 asbestos. The sterilisation is then complete, and the contents 

 of the flask can remain for years without suffering alteration. 

 During the cooling and the indraw the air is partially filtered 

 through the asbestos-plug ; any germs that are carried 

 further are deposited in the lowest part of the bend, or, 

 at the most, do not pass the enlargement of the thin tube, 

 and therefore do not come into contact with the liquid. 

 Hence, it is evident that the lower part of the bent tube 

 must be heated whenever the flask is to be agitated or 

 emptied through the straight tube, without exposing it to 

 contamination. If the flask is to be opened and placed in 

 connection with another flask, this must be effected either in 

 some small germ-free space, or the opening and connecting 

 must be done in a flame. A Bunsen burner is placed 

 directly in front of the operator, the flask to be emptied to 

 the left, and the one that is to receive the liquid or culture 

 to the right, close to the burner. Then the tube of the 

 left-hand flask is opened in the flame by quickly removing 

 the India-rubber tube with its glass stopper ; while the open 

 tube is in the flame, the glass stopper of the flask to the 



1 Chevreul and Hoffmann had previously found that when vessels 

 employed in sterilising liquids are provided with open but bent tubes 

 their contents will remain sterile. Although Chevreul was thus the 

 first to indicate the principle of these flasks, I will not mention them 

 by any other name than that of Pasteur, through whom, indeed, they 

 have obtained a wide application. 



