20 HUMAN BIOLOGY 



7. State also what conditions you have found that hin- 

 der the growth of bacteria. 



D. The effect of antiseptics. — Prepare a pure culture of 

 bacteria in dish No. 13 in the foUowing manner. 

 Heat a dissecting needle on a piece of platinum wire 

 in a hot flame to kill all the germs upon it. When 

 • it cools, touch a colony of bacteria in a Petri dish 

 with the needle-point or wire; carefully raise the 

 cover of dish No. 13 and make several scratches 

 in the agar (the date of the experiment or the num- 

 ber of the room may be scratched in this way). 

 In a similar way prepare dish No. 14 and then pour 

 over the surface some peroxid of hydrogen or other 

 antiseptic solution. When the dishes have been 

 treated as described above, put them in a warm, 

 dark place for several days ? 



1. Describe the preparation of dishes 13 and 14. 



2. In which of the two dishes do you find no colonies of 



bacteria at the end of several days ? 



3. Peroxid of hydrogen is employed in treating wounds. 



How do you know that bacteria are killed by this 

 treatment? 



III. Bacteria as the Friends of Man 



18. Relation of bacteria to soil fertility. — Having dis- 

 cussed somewhat the structure and functions of bacteria, 

 we are now to consider the great importance of these mi- 

 croscopic organisms to human welfare. In the first place, 

 were it not for their never ending activity, all life upon the 

 earth would soon cease to exist. Let us see why this is so. 

 When animals or plants die, their bodies fall upon the ground, 

 and had not these lifeless masses been taken care of, the 

 whole surface of the earth would long since have been covered 

 with a vast number of unburied organisms. All this dead 

 material, however, as we have seen, is food for the countless 



