104 



liACTEKIOLOGY. 



i-emains soli<l up to a tempei-ature of about 45° CI. It is there- 

 fore substituteil for gelatine in llie preparati(jn of a, jell)' for the 

 cultivation of those liaeteria which will onlv grow, or gnnv best, in 

 the incul)ator at the temperature of the blood. It may also be 

 employed at ordinary temperatni'es for liacteria which licpiefy 

 gelatine. The prepar.ition is conducted on much the same piinciples 

 as those alread\' described. Instead, however, of 100 grammes of 

 gelatine, only aljout 20 grammes of agai'-agar are emjiloyed (1-5 to 

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

 in salt water overnight. For the filtr;ition, Haunel is substituted 



for filter-paper, or may 

 lie used in combination 

 with the latter. The 

 hot-water apparatus 

 is invariably employed, 

 lujless, to accelerate 

 the process, the glass 

 funnel and receiver are 

 Ijodily transferre.l to 

 the steam steriliser. If 

 the conical cap cannot 

 lie replaced, cloths laid 

 over the mouth of the 

 steriliser must be em- 

 ployed instead. It may 

 lie necessary to repeat 

 the jjrocess of filtra- 

 tion, but it must not be 

 expected that such a 

 bi-illiant transparency 

 can be obtained as with 

 gelatine. The final 

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

 milk)', it may still he employed. 



A little liquid gradually collects in the tubes, being expi'essed by 

 the contraction of the agar-agar. 



Wort-gelatine is used in stad)-ing 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 foi' special 

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



Fig. 34. — IxcuB.iTOK. 



