78 ELEMENTS OF WATER BACTERIOLOGY 



agar and the separation of the typhoid and colon groups 

 of organisms depends on the greater motility of the 

 former and their tendency to swim out from the colonies 

 and form branch-like processes or turbid zones on a 

 semi-solid medium. The Hiss medium is made up as 

 follows (Hiss, 1902): 



Agar 15 gm. NaCl 5 gm. 



Gelatin 15 gm. Dextrose 10 gm. 



Liebig's meat extract 5 gm. Distilled water 1000 c.c. 

 Reaction i per cent normal. 



The Hesse medium has been used with great success 

 by Jackson and Melia (1909). Its general composition 

 is as follows: 



Agar 5 gm. NaCl 8.5 gm. 



Witte's pepton 10 gm. Distilled water 1000 c.c. 



Liebig's meat extract 5 gm. 

 Reaction i per cent normal. 



Jackson (1909) recommends that the agar used 

 should be dried for half an hour at 105, and under 

 these circumstances 4.5 grams may be used in the 

 formula instead of the 5 grams recommended by Hesse. 

 The medium must be stored in an ice chest with saturated 

 atmosphere. Plates must be made in sufficient dilu- 

 tion to give a few colonies on the plate; and where 

 this is done the typhoid colonies are sharply dis- 

 tinguished from those of B. coli by the fact that they 

 grow to a considerable size, often several centimeters 

 in diameter and show a broad translucent or scarcely 



