CHAPTER IV. 



LIQUID CULTIVATING MEDIA. 



50. In addition to the media described in the preceding chapter, 

 there is a long series of fluid media used in the cultivation of micro- 

 organisms. They are of two main types — firstly, those obtained by 

 infusing animal or vegetable substance in water ; and, secondly, those 

 produced by the artificial commingling of the chemical elements 

 which are present in the bodies of micro-organisms, and which are 

 hence necessary constituents of their food. In some of the latter 

 the quantities of the different constituents have been graduated with 

 exquisite precision to the requirements of the organisms that it is 

 desired to cultivate ; whilst in the former, where fully elaborated 

 natural products are employed, in which doubtless a similar exacti- 

 tude of molecular relations exists, the requisite food material may be 

 obtained by a less complex synthetical process. 



As compared with solid media, three points must be noticed 

 in relation to the growth of micro-organisms in fluids. In fluid 

 media, organisms grow more rapidly than in solid. In the former, 

 however, they do not, as a rule, exhibit such distinctive specific 

 features as in the latter ; but in fluid media there is an opportunity 

 of gauging exactly the chemical requirements and relations of micro- 

 organisms, which does not exist in the case of solids. For the study, 

 then, of the physiological peculiarities of any organism, one would 

 choose a fluid nutrient material ; whilst for that of its morphological 

 relations, solid media would be much better adapted. 



Animal Infusions. 



51. Fresh meat (beef, pork, veal, chicken, &c.), free from fat, and, 

 as far as possible, from fibrous textures, is finely minced and extracted 

 with cold distilled water for twenty-four hours in a cool place, a litre 



