BACILLUS TYPHOSUS. 435 
8. To the filtrate add 1 per cent. of potassium 
iodide. (Use a solution so made that 1 ¢.c. is equiva- 
lent to 1 gramme of the salt.) 
9. Decant into tubes and sterilize. 
One of the most important points in working with 
this medium is that the incubator must be kept at a con- 
stant temperature of from 22° to 24° C. If the plates 
be put away before the gelatin has thoroughly cooled, 
or if the room becomes a very little too warm at any 
time during the colony growth, so as to soften the gel- 
atin, both the typhoid and the colon bacilli will develop 
threads or become oval, and thus the characteristic dif- 
ferentiation will be lost. Care must be taken, also, 
that the room in which the plates are examined be not 
too warm. This causes great inconvenience during the 
summer months in most parts of the United States, 
and requires special methods for keeping a room at the 
proper temperature and keeping the plates cool during 
their examination. 
The iodide of potash prevents the great majority of 
bacteria, especially the liquefiers, from developing; in 
fact, little but colon and typhoid bacilli appear on the 
plates. This is one of the chief advantages of the 
medium in the examination of both water and feces. 
Appearance of the Colonies. The colon is the first to 
develop; the colonies are rough and granular in appear- 
ance and greenish-brown in color; for the greater part 
the colonies are on or near the surface. The typhoid 
develop later, and their colonies usually show the clas- 
sical ‘‘ dew-drop’’ appearance—small, white, gleaming, 
generally without variation in substance, but occasion- 
ally slightly granular. This point causes some trouble 
to one first using the medium, as the young colon colo- 
