NUTRIENT AGAR-AGAE. 109 



the temperature of the mass, is necessary. Agar-agar 

 prepared after the methods just given should pass 

 through a properly folded paper filter at the rate of a 

 litre in from twelve to fifteen minutes. 



Another plan that insures complete solution of the 

 agar-agar without causing the precipitates often seen 

 when all the ingredients are added at once 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 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, and filters more quickly, 

 but, as Schultz has pointed out, it loses at the same time 

 some of its gelatinizing properties. 



Another method by which agar-agar can be easily 

 and quickly melted is by steam under pressure. If 

 the flask containing the mixture of bouillon and agar- 

 agar be kept in the digester or autoclave for ten minutes 

 with the steam under a pressure of about one atmos- 

 phere, as shown by the gauge, the agar-agar will be 

 found at the end of this time completely melted, and fil- 

 tration may then be accomplished with but little difficulty. 



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

 after nitration and before sterilization. The nutritive 

 properties of the media for certain organisms, particu- 

 larly the tubercle bacillus, are increased by the addition 

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



