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LABORATORY EXERCISES IN BACTERIOLOGY. 



in order to give it a solid consistence. The especial value of a solid over a liquid medium 

 rests in the fact that it does not conduce to the free diffusion of the bacteria inoculated 

 upon it, but permits their development into definite and more or less isolated colonies, 

 presenting gross characteristics apt to be lost by diffusion through a liquid, thus facili- 

 tating the recognition of the organisms and their separation into pure cultures. There 

 is comparatively little nutrient value in the gelatine, but it affords an additional value 

 in being liquefied by the proteolytic power of a large class of bacteria, thus giving 

 rise to a broad classification of these microorganisms into the two great classes of 

 liquefying and non-liquefying bacteria. The solid consistence of gelatine is lost at 

 about 30 C., and its usefulness is therefore impaired for bacteria requiring incubation 

 at a temperature above this. This disadvantage may be obviated in some measure 

 by combining with it a small amount of agar. In the preparation of gelatine it should 

 be kept in mind that superheating or too prolonged or too frequent heating, even 

 at the ordinary temperature of the steam-bath (100 C.), may convert the gelatine 



FIG. 26. DISTRIBUTION TO CULTURE TUBE OF LIQUID MEDIUM FROM FLASK A, BY 

 PRESSURE FROM SIPHON FLASK B. 



into paragelatine and destroy its power of congelation ; and caution in this connection 

 is essential for the best results. In its manufacture, if it be necessary to freshly prepare 

 the bouillon, the process may be shortened by neutralizing it immediately after solu- 

 tion of the peptone and salt (litmus paper may in this preliminary neutralization be 

 used as a sufficiently exact indicator), and then adding the gelatine. When prepared, 

 tubed, and sterilized, those tubes intended for surface inoculation should be placed 

 in a slanting position, so that the medium may set with a larger surface for exposure. 

 The congelation is best secured by rapid cooling of the medium on a block of ice, or 

 in the refrigerator, or in a bath of ice-water. 



Exercise 27. (Prepare bouillon as just suggested if not on hand.) 

 Two hundred and fifty cubic centimeters of the bouillon prepared in 

 exercise 25 are measured into a clean stew-pan. Add twenty-five grams 

 of the best sheet gelatine, broken or cut into small pieces, and heat gently 



