82 TEXT-BOOK OF BACTERIOLOGY. 



very useful for some purposes. We dry ordinary bread rye bread 

 is the best at a moderate heat and reduce it to a fine powder. 

 This is put into Erlenmeyer's flasks (about 20 grams in each), and 

 enough distilled water added to make the whole into a uniformly 

 soft, moist pap. This is sterilized an hour a day for three days in 

 the steam generator. 



The bread powder has a slightly acid reaction, and is, therefore, 

 greatly favored by the mould fungi, which, indeed, are generally 

 bred upon it in pure cultures. To make it suitable for bacteria, 

 after the softening with distilled water a solution of soda must be 

 added till alkalescence is produced. 



IV. LIQUID AND SOLID CULTURE MEDIA BEEF-BOUILLON 

 GELATIN AND AGAR-AGAR. 



The solid food media already considered have a decided superi- 

 ority over liquid media, and yet every one of them had a decided 

 fault about it. They were all opaque and therefore unsuitable for 

 direct microscopic examination, which had yielded such valuable 

 revelations in the case of the liquid media. It was a brilliant idea 

 of Koch's that found a means of overcoming this difficulty also : 

 " He changed the liquid media into solid ones by the addition of 

 transparent substances capable of consolidation." This was the 

 grand secret whose discovery was destined to disclose to us a world 

 of new phenomena. 



As liquid food Koch employed beef-bouillon; as consolidating 

 substance he employed gelatin, which gives the liquid solidity while 

 leaving its transparency undiminished. 



Gelatin is a peculiar mass, chiefly obtained from calf s bones or 

 from sinewy and cartilaginous substances. Its chemical composi- 

 tion is not yet exactly known, and probably varies according to cir- 

 cumstances; yet we do know that chondrin and gelatin, with the 

 albuminoids allied to them, are its chief constituents and give it 

 its characteristic properties. The gelatin * which we commonly 

 employ is a French article and is sold in thin transparent leaves. 

 If placed in water or watery fluids for example, in beef -bouillon it 

 swells up and melts, at a temperature of about 24 C., into a homo- 

 geneous solution. It is then perfectly liquid and boils at 100 C., 

 passing readily through filtering membranes, and remains trans- 

 parent and unaltered for any length of time. On the other hand, 

 it returns to the solid state at temperatures under 24 C., and 



* This gelatin is sold under the name of "gold-label gelatin." J. H. L. 



