28 THE STORY OF THE BACTERIA. 



In such a confusing mixture as this the stu- 

 dent finds it no easy task to make out much 

 except differences in form and movement, 

 in the jumble of tiny plants. What he needs 

 to do is to get each species by itself, so that 

 he can cultivate it alone, and find out what it 

 is and does under more simple conditions. 



The device which was formerly in vogue, 

 and which was called the fractional method of 

 separation, was to make some beef-tea which 

 is pretty good food for most bacteria, and 

 putting a little of this in a great many separate 

 little tubes, carefully heat them so as to kill all 

 the bacteria which might by chance have been 

 in the flasks or in the materials from which the 

 beef-tea was made. The investigator also 

 takes a large flask of water and kills by heat- 

 ing all the bacteria which it may contain. Now 

 he dips the end of a fine needle, which has 

 just been heated red-hot to kill the omnipres- 

 ent germs, into the putrefying mixture. He 

 rinses off the invisible amount of material 

 which clung to the needle into the pure water 

 in the large flask, and thoroughly shakes it so 

 that the bacteria which he has put into it may 

 become equally distributed through it. 



