LIQUID MEDIA. 89 



media advocated by Koch. It must not be supposed, 

 however, that the methods of cultivation in liquids 

 are discarded entirely by the German school, for 

 there is no more instructive method than the 

 employment of so-called drop-cultures. For inocu- 

 lation experiments where the presence of gelatine 

 is undesirable, for studying the physiology and 

 chemistry of bacteria and where for any object 

 a rapid growth of micro-organisms is necessary, 

 the employment of liquid media is not only advis- 

 able, but is absolutely necessary. Liquid media 

 comprise two distinct groups natural and artificial. 

 The natural group includes meat broths, blood, 

 urine, milk, and vegetable infusions ; the artificial 

 are solutions built up from a chemical formula 

 representing the essential food constituents. 



NATURAL MEDIA. 



Bouillon. A broth or bouillon of beef, pork, 

 or chicken may be made in the same manner as 

 described for the preparation of gelatine-peptone- 

 broth, with simply omission of the gelatine. After 

 the neutralisation with carbonate of soda solution 

 drop by drop, the flask of broth is placed in the 

 steam steriliser for half an hour at 100 C. A clear 

 liquid results on filtration, which is transferred to 

 plugged sterilised flasks or test-tubes, and sterilisa- 

 tion effected by exposing them in the steam steriliser 



