MATERIAL WITH WHICH TO BEGIN WORK. 245 



more than a 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 cen- 

 tral spot ; and again they may appear as concentric 

 rings ; and if by the methods which have been de- 

 scribed 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 bur- 

 den his mind with names for these organisms ; it is suffi- 

 cient 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 centimetres long) 

 in a sterilized test-tube, and sterilize them by streaming 

 steam for one hour or in the autoclave for fifteen min- 

 utes 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 min- 



