POTATOES AND BLOOD SERUM 133 



pressure in the autoclave. In the latter event one atmos- 

 phere of pressure should be continued for twenty minutes. 

 (See Fig. 18.) 



For some purposes potatoes may be advantageously 

 peeled, sliced into disks of about 1 cm. in thickness, and 

 placed in small glass dishes provided with covers, similar to 

 the ordinary crystallizing dishes. The dish and its con- 

 tents are then sterilized by steam in the usual way. 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 potatoes, 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 organisms commonly 

 found in the ground, the spores of which are among the 

 most resistant known. 



Blood Serum. For ordinary routine work blood serum 

 may be obtained from either the slaughter houses or the 

 antitoxin manufacturers. When from the former the 

 blood that streams from the severed vessels of the throat 

 of the slaughtered animal is collected under as clean con- 

 ditions as possible in large, clean glass museum jars. These 

 are then, with the covers placed upon them, set aside in an 

 ice-chest until coagulation is complete. The serum may then 

 be decanted or pipetted off into flasks and thus transported 

 to the laboratory to be sterilized by the method given 

 below. 



In many localities it is possible to purchase at a small 

 cost normal horse serum in bulk from firms engaged in the 



