NUTRIENT A GAR AGAR. 91 



agar-agar without causing the precipitates that are com- 

 raonlv seen when all the ingredients are added at first 

 and boiled for a long time, is to weigh out the necessary- 

 amount of agar-agar, 10 or 15 grammes, and place this 

 in 1300 or 1400 c.c. of water and boil down over a free 

 flame to 1000 c.c. The peptone, salt, and beef-extract 

 are then added and the boiling again continued until 

 they are dissolved. The clarification with egg-albumin 

 may then be done, and usually the mass filters quite 

 clear and does not show the presence of precipitates 

 upon cooling. If the mixture is positively alkaline, 

 it is not only cloudy, but it filters with difficulty; if it 

 is acid, it is usually quite clear, filters more quickly, but, 

 as Schulze has pointed out, loses at the same time some 

 of its gelatinizing properties. The bouillon should al- 

 ways be neutralized before the agar-agar is added to 

 it, for if tlie bouillon be acid, from tlie acid of the 

 meat, it robs the agar-agar, under the influence of heat, 

 of some of its gelatinizing powers, which cannot be re- 

 gained by subsequent neutralization. 



Another method by which the agar-agar can easily 

 and quickly be melted is by steam under pressure. If 

 the flask containing the mixture of bouillon and agar- 

 agar be kept in the digester or autoclav^e, with the steam 

 under a pressure of about one atmosphere, as shown 

 by the gauge, for ten minutes, the agar-agar will be found 

 at the end of this time completely melted, and filtration 

 may then be accomplished with but little difficulty. 



If glycerin is to be added to the agar-agar, it is done 

 after filtration and before sterilization. The nutritive 

 properties of the media for certain organisms, particu- 

 larly the tubercle bacillus, are improved by the addition 

 of glycerin in the proportion of 5 to 7 per cent. 



