76 BACTERIOLOGY. 



employ the yellow curcuma paper for the detection of 

 alkalinity rather than the red litmus paper. 



Not infrequently the filtered bouillon, neutralized and 

 sterilized, will be seen to contain a fine, flocculent pre- 

 cipitate. This may be due either to excess of alkalinity 

 or to incomplete precipitation of the albumin. The 

 former may be corrected with dilute acetic or hydro- 

 chloric acid, and the bouillon again boiled, filtered, and 

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

 boiling and filtration usually results 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 prefer Liebig's. It is 

 employed 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 advantage of this method is that it takes less time 

 than when the infusion of meat is used, affords a solu- 

 tion of more uniform composition if used in fixed pro- 

 portions, and for general use it answers just as well. 



Nutrient Gelatin. — For the preparation of gela- 

 tin the bouillon is first prepared in exactly the same 

 way as has just been described, except that the neutral- 

 ization takes place after the gelatin has been completely 

 dissolved, which occurs very rapidly in hot bouillon. 

 The reaction of the gelatin as it comes from the manu- 

 factories is usually quite acid, so that a much larger 

 amount of alkali is needed for its neutralization than 

 for other media. The gelatin is added in the propor- 

 tion of 10 to 12 per cent. The complete solution of 

 the gelatin may be accomplished either over the water- 



