THE BIOLOGY OF BACTERIA 21 



remains much more marked than any degeneration which 

 may occur. 



The basis of many of these artificial media is broth. This 

 is made from good lean beef, free from fat and gristle, which 

 is finely minced up and extracted in sterilised water (one 

 pound of lean beef to every looo cc. of water). It is then 

 filtered and sterilised. It will be understood that such an 

 extract is acid. To provide peptone beef-broth, ten grains of 

 peptone and five grains of common salt are added to every 

 litre of acid beef-broth. It is rendered slightly alkaline by the 

 addition of sodium carbonate, and is filtered and sterilised. 

 Glycerine-broth indicates that 6 to 8 per cent, of glycerine has 

 been added after filtration, glucose-broth i or 2 per cent, of 

 grape-sugar. This latter is used for anaerobic organisms. 

 The use of broth as a culture medium is of great value. It 

 is undoubtedly our best fluid medium, and in it may not 

 only be kept pure cultures of bacteria which it is desired to 

 retain for a length of time, but in it also emulsions and mix- 

 tures may be placed preparatory to further operations. 

 Gelatine is broth solidified by the addition of lOO grams of 

 best French gelatine to the litre. Its advantage is twofold : 

 it is transparent, and it allows manifestation of the power of 

 liquefaction. When we speak of a liquefying organism we 

 mean a germ having the power of producing a peptonising 

 ferment which can at the temperature of the room break 

 down solid gelatine into a liquid. Grape-sugar gelatine is 

 made like grape-sugar broth. Agar was introduced as a 

 medium which would not melt at 25° C, like gelatine, but 

 remain soHd at blood-heat (37'5'' C. ; pS'S"" F.). It is a sea- 

 weed generally obtained in dried strips from the Japanese 

 market. Ten to fifteen grams are added to every litre of 

 peptone-broth. Filtration is slow and often difficult, and 

 the result not as transparent as desirable. The former diffi- 

 culty is avoided by filtering in the Koch's steamer or with 

 a hot-water filter, the latter by the addition of the white of 



