88 TEXT-BOOK OF BACTERIOLOGY. 



Instead of gelatin, another substance may be employed, called 

 agar-agar, which is also transparent and capable of solidification. 

 In contrast to gelatin, which is* an animal product and related to 

 the albuminates, agar-agar is a vegetable jelly, obtained from cer- 

 tain species of sea-tangle on the coasts of India and Japan. 



It is sold in the form of dry transparent strips and as a white 

 powder. In both forms it is able to give a completely solid con- 

 sistency, without injuring the transparency. It melts at about 90 

 C., stiffens again at about 40 C., and is not affected by the diges- 

 tive action of the bacteria. 



If, in spite of these advantages, agar solutions have not entirely 

 replaced gelatin, the reason is that the agar is much more difficult 

 to prepare as a culture medium. 



In principle its preparation quite resembles that of gelatin. It 

 is mixed with bouillon, and the mixing is performed as follows : 



First of all, make an ordinary food bouillon, in the manner here- 

 tofore described, with 1,000 grams meat-water, 10 grams peptone, 

 and 5 grams common salt, and then add to the filtered, clear, alka- 

 line liquid, in proportion to its quantity, 1J or at most 2$ of the 

 solid agar-agar. It is not advantageous, as in the case of gelatin, 

 to put the hardening medium in before the filtration, since the 

 conglobated masses of albuminous matter unnecessarily add to the 

 length of the always tedious process of filtering agar-agar. 



For the same reason do not employ more than the above-given 

 proportion of agar-agar; if too much is used, its solidity and con- 

 sistency do not allow it to pass through the filter at all. 



When the agar, cut up into as small pieces as possible, has been 

 put into the bouillon, it is left there for a few hours to swell. 



Then begin to heat the mixture, in order to dissolve the agar. 

 The longer this is continued the more patience must be exercised, 

 the better will the food medium turn out, the more easily will it 

 filter, and the clearer will it be. The boiling must be extended to 

 five or six hours at least; whether it is done over an open flame, or 

 in the steam generator, or in a well-filled water bath is immaterial. 

 When the agar-agar has become so far dissolved that no more 

 coarse portions are perceptible in the bouillon, the reaction must 

 be adjusted. 



If the bouillon was distinctly alkaline a few drops of carbonate 

 of soda solution will suffice, the agar being, in contrast to the gela- 

 tin, almost completely neutral in its reaction. If the liquid after 

 another hour's boiling gives no sign of becoming clear, one can 

 here, as with the gelatin, add the white of a hen's egg. 



Then filter. The great number of proposals which have been 



