536 BACTERIOLOGY. 
and by the fifth day the gelatin is usually all lique- 
fied. The deep colonies, before liquefaction sets in, 
appear as round, granular masses with scalloped mar- 
gins, having a yellowish-green color; the surface col- 
onies have a darker green centre, surrounded by a 
delicate, radiating zone. In stick cultures in gelatin 
liquefaction occurs at first near the surface, in the form 
of a small funnel, and gradually extends downward; 
later the liquefied gelatin is separated from the solid 
part of the medium by a horizontal plane, a greenish- 
_ yellow color being imparted to that portion which is 
in contact with the air. On agar a wrinkled, moist, 
greenish-white layer is developed, while the surround- 
ing medium is bright green; this subsequently becomes 
darker in color, changing to blue-green or almost black. 
In bouillon the green color is produced, and the growth 
appears as a delicate, flocculent sediment. Milk is coag- 
ulated with coincident acid reaction. 
There is some difference of opinion with regard to 
the pigments produced by the bacillus pyocyaneus. 
Gessard’s view is that two pigments are produced by 
this bacillus—one of a fluorescent green and the other 
(pyocyanin) of a blue color. Pyocyanin is soluble in 
chloroform, and may be obtained from pure solution 
in long, blue needles. This pigment, which is thus 
extracted by chloroform, distinguishes the bacillus 
pyocyaneus from other fluorescing bacteria. 
Pathogenesis. This bacillus is very widely distributed 
in nature; it is found on the healthy skin of man, 
in purulent discharges and in serous wound secre- 
tions. Its presence in wounds greatly delays the 
process of repair and may give rise to a general 
depression of the vital powers from the absorption of 
