724 Handbook oj Nature-Study 



since boiling kills these germs. As soon as the milk becomes discolored 

 we know that it is full of bacteria. 



Experiment B shows that bacteria can be transplanted to gelatin, 

 which is a material favorable for its growth. But the point of this experi- 

 ment is to show the child that a soiled finger will have upon it germs 

 which, by growing, cloud the gelatin. They should thus learn the value 

 of washing their hands often or of keeping their fingers out of their mouths. 



Experiment C shows the way the destructive bacteria attack the 

 potato. The discolored spots show where the decay begins, and the odor 

 is suggestive of decay. If a potato thus attacked is put in the bright sun- 

 light the bacteria are destroyed, and this should enforce the moral of the 

 value of sunshine. 



References — The Story of the Bacteria; Dust and its Dangers, M. T. 

 Prudden, Putnam's. Bacteria in. Relation to Country Life, Lipman. 



LESSON CLXXXVII 

 Bacteria 



Leading thought — Bacteria are such small plants that we cannot see 

 them without the aid of a microscope, but they can be planted and 

 will grow. The object of this lesson is to enforce cleanliness. 



Method — Experiment A — The bread used for the mold experiment is 

 likely to develop spots of yellow, red or purple upon it, and cultures from 

 these spots may be used in this lesson as follows: Take some vials, boil 

 them and their corks, and nearly fill them with milk that has been boiled. 

 Take the head of a pin or hairpin, sterilize the point by holding in a flame, 

 let it cool, touch one of the yellow spots on the bread with the point, being 

 careful to touch nothing else, and thrust the point with the bacteria on it 

 into the milk ; then cork the vials. 



Experiment B — Prepare gelatin as for the table but do not sweeten. 

 Pour some of this gelatin on clean plates or saucers. After it has cooled 

 let one of the children touch lightly the gelatin in one saucer for a few 

 seconds with his soiled finger. Note the place. Ask him to wash his 

 hands thoroughly with soap and then apply a finger to the surface of the 

 gelatin in the other plate. Cover both plates to keep out the dust and 

 leave them for two or three days in a dark place. The plates touched by 

 the soiled finger will show a clouded growth in the gelatin ; the other plate 

 will show a few irregular, scattered growths or none. 



Experiment C — Take a slice of boiled potato, place in a saucer, leave it 

 uncovered for a time or blow dust upon it, label with date, then cover with 

 a tumbler to keep from drying and place in a cool, somewhat dark place. 



The pupils should examine all these cultures every day and make the 

 following notes : 



Experiment A — How soon did you observe a change in the color of the 

 milk? How can you tell when the milk is full of the bacteria? How do 

 you know that the bacteria in the milk was transplanted by the pin? 



Experiment B — Can you see that the gelatin is becoming clouded 

 where the soiled finger touched it ? This is a growth of the bacteria which 

 were on the soiled finger. 



Experiment C — What change has taken place in the appearance of the 

 slice of potato ? Are there any spots growing upon it ? What is the odor? 



