BACTERIA 



251 



Prepare a dozen bottles or tumblers in this way. We 

 must now sterilize them, that is, kill the bacteria in them. 

 Place them in a tin pail on the bottom of which is a 

 small pan turned upside down and pierced with nail 

 holes, as shown in 

 figure 152. Pour in 

 water until it stands 

 an inch or so deep 

 on the bottom. 

 Cover the pail with 

 a pan. Now let the 

 water boil in the 

 pail for at least half 

 an hour, and when 

 cool take out the 

 tumblers. Take 

 care that the cotton 



FIG. 151. Two bottles containing (a) bacteria 

 which need air in order to grow, (6) bacteria 

 which cannot grow when air is present. 



We are now ready to make 



is not wetted by 

 drops falling from 

 above or by water 

 boiling up from below. 

 some experiments. 



How to grow bacteria. Instead of potato we may 

 use gelatin, which can be made with only a little more 

 trouble and which is very much better for most experi- 

 ments. Ordinary gelatin such as is used in cooking is 

 placed in about seven times its weight of water. Place 

 it in a pail, as shown in figure 152, with some water in 



