TO ILLUSTRATE EXTRANEOUS CONTAMINATION 271 



B. HAIRS FROM COW 



Method. 1. Select two hairs from the back of the cow 

 where the usual or natural clean- 

 liness exists and two from the 

 hip stained with manure. By 

 means of sterile forceps place 

 them in sterile Petri dishes. 



2. One of each kind, that is, 

 one from the back and one from 

 the hip, place in 10 c.c. of a 

 sterile salt solution, as under A. 



3. Shake thoroughly, then 

 plate 1 c.c. of this suspension in 

 litmus lactose agar. 



4. Embed each of the two 

 remaining hairs in the litmus 

 lactose agar after pouring the 



liquefied agar into a sterile Petri dish. These hairs should 

 be placed in the agar just before solidifying by means of 

 sterile forceps. 



FIG. 59. Bad. bulgaricum 

 colony, x75. (Orig. Nor- 

 thrup.) 



C. OTHER SUBSTANCES 



Method. Study straw, hay, dung, etc., in a similar 

 manner. Instead of using dilution flasks containing 10 c.c. 

 it will be more desirable to use 100 c.c. 



In the case of dung, a particle smaller than the head 

 of a pin should be added to 100 c.c. for suspension, and 

 in case of straw and hay very small segments unless they 

 are very clean. 



Note. A sufficient number of such substances should be studied 

 to familiarize the student with the amount of contamination which 

 may take place from these sources. 



