84 BACTERIOLOGY. 



i. e., from 15 to 20 c.c. less than the amount needed 

 to bring the reaction up to the neutral point. 



Not infrequently the filtered bouillon, neutralized 

 and sterilized, will be seen to contain a fine, flocculent 

 precipitate. This may be due either to excess of alka- 

 linity or to incomplete precipitation of the albumin. 

 The former may be corrected with dilute acetic or 

 hydrochloric acid, and the bouillon again boiled, filtered, 

 and sterilized; or, if due to the latter cause, subsequent 

 boiling and filtration usually result in ridding the 

 bouillon of the precipitate. 



Another modification now generally employed is the 

 use of meat-extracts instead of the infusion of meat. 

 Almost any of the meat-extracts of commerce answer 

 the purpose, though we usually employ Liebig's. It 

 is used in the strength of from two to four grammes to 

 the litre of water. Peptone and sodium chloride are 

 added as in the bouillon made from the meat-infusion. 

 The advantages of meat-extract are: it takes less time; 

 affords a solution of more uniform composition if used 

 in fixed proportions, and in general use gives results 

 that are equally as satisfactory as those obtained from 

 the employment of infusion of meat. 



Nutrient Gelatin. — For the preparation of gelatin 

 the bouillon is first prepared in exactly the same way 

 as has just been described, except that the neutralization 

 takes place after the gelatin has been completely dis- 

 solved, which occurs very rapidly in hot bouillon. The 

 reaction of the gelatin as it comes from the manufac- 

 tories is frequently quite acid, so that a much larger 

 amount of alkali is need for its neutralization than 

 for other media. It is possible, however, to obtain 

 from the makers an excellent grade of gelatin from 



