THE SPORE-BEARERS 55 



division so that they form colonies that are readily visible, and these usually 

 have a characteristic color, shape, or other peculiarity. Bacteria are 

 capable of such rapid reproduction that it is calculated that one of them 

 growing without check would develop a mass of them as big as the earth in 

 a week's time. 



Cultures. Very thoroughly clean six test tubes and two petrie dishes or 

 four saucers, two large and two small. Obtain some grape gelatin, the 

 sort that is sold at the grocery stores for puddings. Melt it according to 

 directions as if preparing it for pudding. Add a quarter-tablespoon (level) 

 of soda to neutralize the acid and make it slightly alkaline. Most bacteria 

 grow best in an alkaline medium. Fill the test tubes one-fourth full and 

 plug them with absorbent cotton. Fill the bottoms of the petrie dishes 

 or the two small saucers, covering the dishes with their covers or the saucers 

 each with an inverted large saucer. Put these all in the sterilizer and steam 

 twenty minutes. Then cool, standing the test tubes in a much-inclined 

 position so that the gelatin will set 'at a slant from near the mouth of the 

 test tube to near the bottom on the other side. 



Planting bacteria. Catch a fly and let him crawl around on the gelatin 

 in one slant tube, then release him, replugging the tube. Label it so that 

 you will know which one was used for this experiment. With a toothpick 

 scrape the teeth and smear the scrapings on the gelatin surface in another 

 slant tube, then plug it again and label. Leave one of the slant tubes open 

 in the schoolroom for fifteen minutes before school and another for fifteen 

 minutes near the close of the session. Plug each at the end of the time and 

 label. The sixth tube will be left just as it came from the sterilizer for 

 comparison. 



Let some pupil with rather dirty fingers press his thumb on the gelatin 

 in one of the saucers (or petrie dishes). Cover it immediately; then let 

 him wash his hands well and come and press the same thumb on the gelatin 

 in the same saucer but on a different spot; cover promptly. The second 

 saucer will be left as it came from the sterilizer. Set these all aside where 

 they can be at least at room temperature. Examine daily to see if colonies 

 of bacteria appear. Each spot is a colony and the number of spots that 

 appear will give some idea of the number of bacteria or mold spores that 

 were deposited on the surface of the gelatin. 



