44 PURE CULTURE. OF BACTERIA 



In a cubic centimetre or even a drop of the liquid 

 there will usually be hundreds or thousands of bacteria, 

 numbers far too large to be effectually dealt with. A 

 small portion of the milk or other liquid is, therefore, 

 transferred to a large bulk of water, which must, of 

 course, be first rendered free from living bacteria by boiling 

 for some time, and the whole shaken to distribute the 

 organisms evenly. (If the amount taken is a cubic centi- 

 metre containing, say, 1000 bacteria, and this is placed 

 in 99 cubic centimetres of water, the diluted mixture 



will now contain on an average 

 ten of these organisms in every 

 cubic centimetre, a number 

 more easily dealt with.) A cubic 

 centimetre or smaller quantity 

 is now taken from such a mix- 



r iG. 16. .retri dish. 



ture and introduced into a test- 

 tube containing 8 to 10 cubic centimetres of sterilized 

 liquid nutrient medium or solution of substances upon 

 which bacteria can feed and multiply, to which has been 

 added a small percentage of gelatine, agar-agar, or other 

 material possessing the property of solidifying to a jelly 

 at ordinary temperatures. The nutrient gelatine medium 

 is kept sufficiently warm to render it liquid, and the 

 whole is carefully shaken to separate and distribute the 

 bacteria which have been added to it. The contents of the 

 test-tube are then poured into a glass Petri dish (Fig. 1 6) or 

 on to the surface of a glass plate : the medium is allowed 

 to cool and solidify, protected from dust. In the thin 

 solid layer of nutrient jelly thus obtained the bacteria 

 are imprisoned, and although they can grow and flourish 

 they cannot move far or become mixed with each other 

 as would have been the case if the nutrient medium had 



