74 METHODS USED IN THE STUDY OF BACTERIA 



Gelatin is frequently modified by the addition of such sub- 

 stances as dextrose, lactose ; and a medium formerly exten- 

 sively used for the isolation of B. typhosus was a potato- 

 water gelatin containing potassium iodide (Eisner). 



A gar. Agar is a culture medium made by adding a small 

 per cent of the vegetable agar-agar to bouillon. Agar-agar 

 is a seaweed that grows in the Pacific Ocean, and it is known 

 as Bengal isinglass, or Ceylon moss. It occurs in the market 

 as transparent strips, or sticks, and as a white powder. It 

 differs from gelatin in being a carbohydrate simply, and not a 

 protein. It is neutral in reaction and melts at about 99 C. 

 and solidifies at about 40 C. Its chief value in bacteriology 

 comes from its high melting point, and also from the fact that 

 it is not acted upon by ferments which liquefy gelatin, so that 

 it may be used to grow bacteria which need a higher tem- 

 perature than gelatin would remain solid at and to grow 

 bacteria which would liquefy gelatin. From i to 2 per cent 

 of the agar-agar is used. The preparation of agar offers some 

 difficulty to the beginner on account of its difficult solubility 

 and its high solidifying point, which makes it difficult to filter ; 

 but if the agar is gotten into a solution first in the water, and 

 the other ingredients added later, it may be prepared very 

 readily and even filtered through filter paper at the ordinary 

 room temperature. For filtration, however, absorbent cotton 

 is frequently used and the process is sometimes hastened by 

 the use of air suction. 



Agar is variously modified by the addition of sugars, glyc- 



