212 



FIRST YEAR SCIENCE 



Experiment 113. Place a slice of freshly boiled potato in each 

 of five clean 4-ounce wide-mouth bottles. Close the mouths of the 

 bottles with loose wads of absorbent cotton. Place four of these 

 bottles in a sterilizer and sterilize for half an hour. Allow one 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 boiling.) A reli- 

 able inexpensive sterilizer is* 

 the pressure cooker shown in 

 Figure 104. 



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 absorbent-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 steri- 

 lized 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. Keep the fourth bottle just as it was 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 appear- 

 ing on the pieces of potato are bacteria colonies. 



99. Bacteria. The nitrogen-fixing bacteria were consid- 

 ered, to some extent, under soil, but, as these soil bacteria 



Fig. 104. 



