42 BACTERIOLOGY 



stances to the gelatine, so as to meet individual require- 

 ments in cultivating micro-organisms, and accordingly the 

 greatest multiplicity of admixtures have been recommended 

 by approved investigators. For example, Koch uses mixtures 

 of gelatine with blood-serum', aqueous humour, infusion of 

 hay and of wheat, decoction of horses' dung, and decoction of 

 plums. 



Miquel uses, instead of meat bouillon, a solution of 40 

 parts peptone, 10 parts common salt, and 1 part carbonate 

 of potash in 1,000 parts of water, to which the further addi- 

 tion of 4 parts of gelatine can be made. 



Holtz has devised a potato -gelatine, for use in growing 

 typhoid bacilli. The potatoes are grated and squeezed 

 through a straining-cloth, the liquid which flows away is 

 allowed to stand for twenty-four hours, and is then boiled 

 with 10 per cent, of gelatine. 



Preparation of urine gelatine. A very cheap and easily- 

 prepared gelatine is the urine gelatine recommended by 

 Heller. Urine is caught in sterilised vessels, and its 

 specific gravity having been brought to 1010 by dilution 

 with sterilised water, it is rendered feebly alkaline with soda 

 solution and filtered. After 1 per cent, of peptone, ^ per 

 cent, of common salt, and 5 to 10 per cent, of gelatine have 

 been added, it is next boiled and filtered, and the fluid so 

 obtained is poured into test- glasses and sterilised, a single 

 sterilisation being said to be sufficient. This process can 

 be modified by filtering the urine through animal charcoal 

 before diluting it with water, in order to remove part of the 

 urinary colouring matter. 



Preparation of nutrient agar. Agar-agar is a vegetable 

 jelly procured from different algae growing in the East 

 Indies and Japan, and was introduced into bacteriology by 

 Hesse because of its distinctive property of remaining in 

 the solid state at 40 C., and only melting completely at 



