EXPOSURE AND CONTACT 325 



smooth, non-characteristic surface; while in others minute, 

 sometimes regularly arranged tiny corrugations may be 

 observed. In one colony they may appear as tolerably 

 regular lines, radiating from a central spot; and again they 

 may appear as concentric rings; and if by the methods which 

 have been described we obtain from these colonies their 

 individual components in pure culture, we shall see that 

 this characteristic arrangement in folds, radii, or concentric 

 rings, or the production of color, is characteristic of the 

 growth of the organism under the conditions first observed, 

 and by a repetition of those conditions may be reproduced 

 at will. 



So much for the simplest naked-eye experiment that can 

 be made in bacteriology, and which serves to furnish the 

 beginner with material upon which to commence his studies. 

 It is not necessary at this time for him to burden his mind 

 with names for these organisms; it is sufficient for him to 

 recognize that they are of different species, and that they 

 possess characteristics which will enable him to differentiate 

 the one from the other. 



Exposure and Contact. Make a number of plates from 

 bits of silk used for sutures, after treating them as follows: 



Place some of the pieces (about 5 centimeters long) in a 

 sterilized test-tube, and sterilize them by streaming steam 

 for one hour or in the autoclave for fifteen minutes at one 

 atmosphere pressure. At the end of the sterilization remove 

 one piece with sterilized forceps and allow it to brush against 

 your clothing, then make a plate from it; draw another 

 piece across a dusty table and then plate it. Suspend three 

 or four pieces upon a sterilized wire hook and let them hang 

 for twenty minutes free in the air, being sure that they 

 touch nothing but the hook; then plate them separately. 



