APPLICATION OF THE METHODS OP 

 BACTERIOLOGY. 



CHAPTER XVI. 



To Obtain Material with Which to Begin Work. 



EXPOSE to the air of an inhabited room a slice of freshly 

 steamed potato or a bit of slightly moistened bread upon a 

 plate for about one hour. Then cover it with an ordinary 

 water-glass, place it in a warm spot (temperature not to 

 exceed that of the human body 37.5 C.), and allow it 

 to remain undisturbed. In from twenty-four to thirty-six 

 hours there will be seen upon the cut surface of the bread 

 or potato small, round, oval, or irregularly round patches 

 which present various appearances. These differences in 

 macroscopic appearance are due in some cases to the presence 

 or absence of color; in others to a higher or lower degree 

 of moisture; in some instances a patch will be glistening 

 and smooth, while its neighbor may be dull and rough or 

 wrinkled; here will appear an island regularly round in 

 outline, and there an area of irregular, ragged deposit. All 

 these gross appearances are of value in aiding us to distin- 

 guish between these colonies for colonies they are, and 

 under the same conditions the organisms composing each of 

 them will always produce growth of exactly the same ap- 

 pearance. It was just such an observation as this that sug- 

 20 ( 305 ) 



