120 BACTERIOLOGY 



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 routing work blood-serum may be obtained 

 from either the slaughter houses or the antitoxin manufac- 

 turers. When from the former the blood that streams from 

 the severed vessels of the throat of the slaughtered animal 

 is collected under as cleanly conditions 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 

 manufacture of antitoxins and other biological products. 

 This serum, obtained under aseptic precautions, has ob- 

 viously an advantage, and has in our hands proven entirely 

 satisfactory for routine work. 



In either case the serum is to be decanted into clean, 

 sterile test-tubes provided with cotton plugs, after which 

 it must be immediately sterilized. For this purpose the 



