BIOLOGIC CHARACTERS 183 



of the chromogenic bacteria find the potato a most suitable 

 pabulum on which to form their pigment, the pigment 

 formed on this medium having at times an especial bril- 

 liancy. Note in detail all the changes that occur in the growth 

 on successive days. 



Growth in Bouillon. — Observe whether the fluid shows 

 turbidity or not, as well as the extent and distribution of 

 this alteration. Note whether any sediment is being formed, 

 as well as the nature and amount of such sediment. Does 

 the organism form a definite growth (pellicle or scum) on 

 the surface of the bouillon? What is the character of the 

 pellicle? Is it readily dislodged, and, when dislodged, is 

 it replaced by a new pellicle? Note whether the color of 

 the medium has become altered. Note the manner in which 

 the appearance of the culture changes on several successive 

 days. 



Growth in Litmus-milk. — Observe the nature of the growth 

 in litmus-milk. Has the reaction of the medium become 

 altered? To what is such alteration attributable? Note 

 whether there is precipitation of casein. Record the extent 

 and rapidity with which this alteration takes place, as well 

 as the reaction of the fluid while the change is being pro- 

 duced. Is there any evidence of the subsequent liquefaction 

 of the precipitated casein? Has the litmus been altered in 

 any manner except as shown by altered reaction of the 

 medium? In what part of the tube has such alteration of 

 the litmus commenced? If the litmus has been decolorized, 

 is it possible to restore its color by the admixture of air with 

 the fluid? Note the order in which the appearance of the 

 medium ch'anges on successive days. 



Growth in Special Media. — ^The special culture-media may 

 be employed to ascertain additional biologic characters of 



