86 BACTERIOLOGY. 



to twenty-five minutes. When finished they have the 

 appearance seen in Fig. 15, except that there is no 

 growth upon the surface as is shown in the cut. 



Care must be given to the sterilization of potatoes, 

 because they always have adhering to them the or- 

 ganisms commonly found in the ground, the spores of 

 which are among the most resistant of all known 

 organisms. The so-called " potato bacillus " is one of 

 this group ; it is an organism which not infrequently 

 is more or less of an obstacle to the work of the 

 beginner. 



Blood-Serum. — Blood-serum requires special care 

 in its preparation ; it is desirable under all conditions 

 to reduce the unavoidable contamination, which to a 

 certain extent occurs during manipulation, to its min- 

 imum degree. 



It is possible to collect serum from small animals and 

 in small quantities under such precautions that it is 

 perhaps not contaminated, but ordinarily for laboratory 

 purposes a larger quantity is needed, so that the 

 slaughter-houses form the sources from which it is 

 usually obtained, and here a certain amount of contami- 

 nation is unavoidable, though its degree may be limited 

 by proper precaution. The animal from which the 

 blood is to be collected should be drawn up to the ceil- 

 ing by the hind legs, the head should be held well back, 

 and with one pass of a very sharp knife the throat 

 should be completely cut through. The blood which 

 will be spurting from the severed vessels should be col- 

 lected in large glass jars which have been previously 

 cleaned, disinfected, and all traces of the disinfectant 

 removed with alcohol and finally ether. The latter 

 evaporates very quickly and leaves the jar quite dry. 



