192 BACTEEIA. 



Sprinkling a little camphor or chloride of lime about the 

 corners of a sick room may have a good moral effect on 

 patient and attendants, but would only make an intelli- 

 gent microbe with a sense of humor smile derisively. In 

 order to study these organisms and find out the manner 

 of their growth, their various properties, whether patho- 

 genic or not, and to determine their species, it is neces- 

 sary to cultivate them in some medium in which they 

 will grow rapidly, and then we must be able to separate 

 one species from another, so as to obtain them in " pure 

 cultures." We can, if we choose, use simply beef broth 

 for this purpose. The bouillon is put into tubes which 

 are then plugged with cotton. Now, although the air 

 can pass freely through the cotton, and the beef tea can 

 slowly evaporate through it, no bacteria can get through. 

 The cotton acts as a very efficient filter, so that when once 

 the liquid is sterilized no putrefaction can occur in it, no 

 matter how long it may be kept, provided that the cot- 

 ton plug remains undisturbed. Then the tubes are steril- 

 ized with streaming steam on three successive days for 

 ten to fifteen minutes each day. The principle is this : 

 although the first steaming will kill all the bacteria in 

 the tubes it will not kill the spores. At the end of twenty- 

 four hours most of the spores will have developed into 

 mature bacteria, which are killed by the second steaming. 

 The few remaining spores in turn develop and are de- 

 stroyed on the third day. The fluid is then perfectly 

 sterile and ready for the growth of the plants. A needle 

 which has been sterilized by passing through a flame is 

 dipped into material containing bacteria and then, after 

 rapidly removing the cotton plugfrom the tube,the needle 

 is plunged into the tube and the plug replaced as soon as 

 the needle is withdrawn. In a day or two the bouillon is 

 crowded with organisms developed from those carried in 

 on the needle and a portion of it can be dropped on a 

 cover glass, dried, stained and examined under the mi- 



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