CULTURE-MEDIA. 63 



tions in granite or enamel-ware vessels over a large Bunsen 

 or rose-burner. Neutralize with sodium hydroxide solution 

 (see page 59). Litmus-paper or titration may be used for 

 testing. 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 thoroughly. 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 filtration 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 sterihze 

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

 fifteen minutes on each of three consecutive days. Gelatin is 

 injured by too prolonged boihng and loses its solidifying quali- 

 ties. The remarks on pages 60 to 62 with regard to the use 

 of beef and the titration method for the preparation of bouiUon 

 apply equally to gelatin. 



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

 layers of absorbent cotton placed inside of the moistened 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 coagulated 

 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 hydrochloric acid if necessary. 

 It may be well to stir in another egg to entangle the opaque 

 particles ; then to boil a second time and filter. 



The medium is sometimes modified by adding to it other 

 substances, as sugar, glycerin, etc. The solidifying property 



