CULTURE-MEDIA. 75 



Dissolve the ingredients in the water, stirring actively 

 to prevent burning at the bottom. It is best to conduct the 

 operations in granite- or enamel-ware vessels over a large 

 Bunsen or rose-burner. Neutralize with sodium hydroxide 

 solution (see page 71). The reaction at the beginning will 

 usually be found to be quite acid. Allow the mixture to cool 

 until below 60 C, and add the whites of one or two eggs 

 which have been beaten up with a little water; stir in thor- 

 oughly. Heat the mixture to the boiling-point; stir at the 

 bottom to prevent burning and at the same time avoid as far 

 as possible breaking the coagulum of egg-albumen which 

 forms at the surface. Boil for ten minutes. Filter while 

 hot. The nitration may be done through folded filter-paper 

 which has been moistened. It is well to fasten a piece of 

 coarse cheese-cloth over the top of the funnel to catch the 

 large particles of coagulated albumen. Place in suitable 

 tubes or flasks plugged with cotton, and sterilize once in the 

 autoclave, or, preferably, in the steam sterilizer for fifteen 

 minutes on each of three consecutive days. Gelatin is in- 

 jured by too prolonged boiling and loses its solidifying 

 qualities. Neutralization may be with litmus paper or by 

 titration. The remarks on pages 72 to 74 with regard to 

 the use of fresh beef and the titration method for the prepa- 

 ration of bouillon apply equally to gelatin. 



Instead of filter-paper, some prefer to filter through sev- 

 eral layers of absorbent cotton placed inside of the moist- 

 ened glass funnel, the top of which is covered with coarse 

 cheese-cloth. This expedient answers very well. 



If the product appears cloudy after it has been sterilized, 

 it may be that the egg-albumen was incompletely coagu- 

 lated in the first place or that the reaction has been made 

 too alkaline. In any case it will be desirable to melt it 

 and filter a second time, correcting the reaction with hydro- 

 chloric acid if necessary. It may be well to stir in another 

 7 



