94 



BACTERIOLOGY. 



ders of potato are now to be left in running water over 

 night, otherwise they will be very much discolored by the 

 sterilization to which they are to be subjected. At the 

 end of this time they are placed in previously prepared 

 test-tubes, one piece in each tube, with the slanting sur- 

 face up, the cotton plugs of the tubes replaced, and they 

 then to be sterilized in the steam for fifteen to 



are 



Fig. 18. 



twenty minutes on each of three successive days. 

 Or the entire sterilization may be accomplished in 



the autoclave, with the steam under a pressure of one 

 atmosphere, by a single exposure of twenty 

 to twenty-five minutes. When finished 

 they have the appearance seen in Fig. 18, 

 except that there is no growth upon the 

 surface as is shown in the cut. 



For some purposes potatoes may be ad- 

 vantageously peeled, sliced into disks of 

 about 1 cm. in thickness, and placed in 

 small glass dishes provided with covers, 

 similar to the ordinary Petri dishes. The 

 dish and its contents are then sterilized by 

 steam in the usual way (method suggested 

 by von Esmarch). By this plan a relatively 

 large area for cultivation is obtained. 



Potatoes may also be boiled, or steamed, 

 and mashed, and the mass placed in covered 

 dishes, test-tubes, or flasks, and sterilized. 

 By this method one obtains in the mass a 

 mean of the composition of the several pota- 

 toes, or bits of potatoes, used in making it, 



an advantage where uniformity is desired. 



Care must be given to the sterilization of potatoes, 



because they always have adhering to them the organ- 



Potato in test 

 tube. 



