62 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, fiocculent 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. 



Nutrient Gelatin. ^-Por 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 thau for 

 other media. The gelatin is added in the proportion of 

 from 10 to 12 per cent. The complete solution of the 

 gelatin may be accomplished either over the water-bath, 

 in the steam sterilizer, or over a free flame. If the latter 

 method is practised, care must be given that the mixture 

 is constantly stirred to prevent burning at the bottom 

 and consequent breaking of the flask, if a flask is em- 

 ployed. 



For some time it has been our practice to use, for the 

 purpose of making both gelatin and agar, enamelled iron 

 saucepans instead of glass flasks ; by this means the free 

 flame may be employed without danger of breaking the 

 vessel, and, with a little care, without fear of burning 

 the media. Under any conditions it is better to protect 



