104 



BACTERIOLOGY. 



remains solid up to a temperature of about 45 C. It is there- 

 fore substituted for gelatine in the preparation of a jelly for the 

 cultivation of those bacteria which will only grow, or grow best, in 

 the incubator at the temperature of the blood. It may also be 

 employed at ordinary temperatures for bacteria which liquefy 

 gelatine. The preparation is conducted on much the same principles 

 as those already described. Instead, however, of 100 grammes of 

 gelatine, only about 20 grammes of agar-agar are employed (1'5 to 

 2 per cent.), and to facilitate its solution it must be allowed to soak 

 in salt water overnight. For the filtration, flannel is substituted 



for filter- paper, or may 

 be used in combination 

 with the latter. The 

 hot-water apparatus 

 is invariably employed, 

 unless, to accelerate 

 the process, the glass 

 funnel and receiver are 

 bodily transferred to 

 the steam steriliser. If 

 the conical cap cannot 

 be replaced, cloths laid 

 over the mouth of the 

 steriliser must be em- 

 ployed instead. It may 

 be necessary to repeat 

 the process of filtra- 

 tion, but it must not be 

 expected that such a 

 brilliant transparency 

 can be obtained as with 

 gelatine. The final 



result, when solid, should be colourless and clear; but if slightly 

 milky, it may still be employed. 



A little liquid gradually collects in the tubes, being expressed by 

 the contraction of the agar-agar. 



Wort-gelatine is used in studying the bacteria of fermentation. 

 It is made by adding from 5 to 10 per cent, of gelatine to beer- wort. 

 Glycerine Agar-agar. This is prepared by adding 5 per 

 cent, of glycerine to nutrient agar-agar, after the boiling and before 

 the filtration, and other modifications can be made for special 

 purposes by the addition of grape-sugar or of gelatine. 



FIG. 34. INCUHATOR. 



