PLANTS THAT CHANGE FOOD 439 



the bottles with loose wads of absorbent cotton. Place five of 

 these bottles in a sterilizer and sterilize for half an hour. Allow 

 the sixth bottle to remain unsterilized. (A sterilizer can be made 

 by taking a covered tin pail and putting into the bottom of it a 

 bent piece of tin with holes punched in it to act as a shelf on which 

 to put the bottles. A shallow tin dish with holes in it is good for 

 the shelf. There must be holes so that the steam will not get under 

 the shelf and upset it. Fill the sterilizer with water to the top of 

 the shelf and place the bottles on the shelf. Keep the water boil- 

 ing.) A reliable, inexpensive sterilizer is the pressure cooker shown 

 on page 126. 



Take the bottles out and allow them to cool. Remove the cotton 

 from one of them for several minutes and then replace. Run a 

 hat pin two or three times through the flame of a Bunsen burner to 

 sterilize it and place it in the water of a vase which has had flowers 

 in it for some time. Carefully pulling aside the edge of the absorb- 

 ent-cotton stopper in the second bottle, insert the pin and place 

 a drop of the vase water on the surface of the piece of potato. 

 After having sterilized the pin again, rub it several times over the 

 moistened palm of the hand and then, using the same precautions 

 as before, scratch the potato in the third bottle. Put a fly in the 

 fourth bottle, using the same precautions. Keep the fifth bottle 

 just as it w T as taken from the sterilizer as an indicator, that is, 

 to see whether the bottles were thoroughly sterilized. Put all 

 of the bottles away in a warm place and observe them each day for 

 several days. The spots appearing on the pieces of potato are 

 bacteria colonies. 



Since bacteria and fungi cause the " spoiling " of food, 

 and since certain bacteria develop poisons called 

 ptomaines which make the eating of the food infected very 

 dangerous, it is necessary that food be protected as far as 

 possible from bacteria and that their growth be checked. 

 Food should never be handled except with clean hands; 

 it should be most carefully protected from dust and flies 

 and kept in a clean, cool place. Most bacteria do not thrive 

 where it is cold. 



