374	PATHOGENIC BACTERIA.
ical or of a whetstone form, and by transmitted light are
darker, more opaque, and less refractive than the typhoid
colonies. By reflected light, to the unaided eye they are
pale yellow. The surface-colonies are large, round, irreg-
ularly spreading, and are brown or yellowish-brown in
color. Hiss claims that by the use of these reagents the
typhoid bacillus can be readily detected in typhoid stools.
When transferred to gelatin puncture-cultures the ba-
cilli develop along the entire track of the wire, with the
formation of minute confluent spherical colonies. A
small thin whitish layer develops upon the surface near
the center. The gelatin is not liquefied, but sometimes
is slightly clouded in the neighborhood of the growth.
The growth upon the surface of obliquely solidified gela-
tin, agar-agar, or blood-serum is not very luxuriant. It
forms a thin, moist, translucent, non-characteristic band
with smooth edges.
Upon potato a growth formerly regarded as character-
istic takes placd When the potato is inoculated and
stood in the incubating-oven, no growth can be detected
at the end of the second day, unless the observer be
skilled and the examination thorough. If, however, the
medium be touched with a platinum wire, it is discovered
that its entire surface is covered with a rather thick, in-
visible layer of a sticky vegetation which the microscope
shows to be made up of bacilli. No other bacillus gives
the same kind of growth upon potato. Unfortunately, it
is not constant, for occasionally there will be encountered
a typhoid bacillus which will show a distinct yellowish
or brownish color. The typical growth seems to take
place only when the reaction of the potato is acid.
In bouillon the only change produced by the growth of
the bacillus is a diffuse cloudiness.
In milk a slight and slow acidity is produced. The
growth in milk is not accompanied by coagulation.
The chief hindrance to the ready isolation of the
typhoid bacillus is the closely-allied Bacillus coli com-
munis. This organism, being habitually present in the