THE STORY OF THE BACTERIA. 33 



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, bacteria 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 sterilized platinum wire, some 

 of the bacteria are introduced into the gelatin, 

 the cotton plug being removed for an instant 

 for this purpose. Being transparent, the gela- 

 tin permits us to see from the sides as well as 



