34 The Story of the Bacteria 



of growth of bacteria on potatoes is limited, 

 because as the potato is opaque we can see 

 only the surface of the colony; and, further- 

 more, not all the bacteria grow well on pota- 

 toes, and some do not grow upon them at all. 

 So the next step is to make some transparent 

 solid substance which shall be a suitable soil 

 for bacterial growth. One of the most common 

 and useful substances for this end is a 10 per 

 cent, solution of gelatin which is mixed with 

 beef tea, pepton, and a little common salt, 

 and then made neutral or slightly alkaline by 

 carbonate of soda. This mixture, carefully 

 heated so as to destroy all bacteria which 

 might be present in its ingredients, is filled 

 into ordinary glass test-tubes which have been 

 sterilized by a high temperature. These are 

 filled about one third full of the gelatin mixture, 

 and the opening is stopped by a plug of cotton 

 batting. Through a long plug of cotton, bac- 

 teria cannot pass ; the air can enter and leave 

 the tube, but all bacteria are caught by the 

 fibres of the cotton. After the gelatin has 

 become cool and solid, by means of a steril- 

 ized platinum wire, some of the bacteria are 



