METHODS OF ISOLATION. 73 



bacteria when found together under other conditions ? 

 It was plain that the organisms were to be distinguished 

 primarily, the one from the other, only by the structure 

 and general appearance of the colonies growing from 

 them, for by their morphology alone this is impossible. 

 What means could be devised, then, for separating the 

 individual members of a mixture in such a way that 

 they would remain in a fixed position, and be so widely 

 separated, the one from the other, as not to interfere 

 with the production of colonies of characteristic appear- 

 ance, which would, under the proper conditions, develop 

 from each individual cell ? 



If one take in the hand a mixture of barley, rye, 

 corn, oats, etc., and attempt to separate the mass into 

 its constituents by picking out the different grains, much 

 difficulty is experienced ; but if the handful of grain be 

 thrown upon a large flat surface, as upon a table, the 

 grains become more widely separated and the task is 

 considerably simplified; or, 'if sown upon proper soil, 

 the various grains will develop into growths of entirely 

 different external appearance by which they can readily 

 be recognized as unlike in nature. Similarly, if a test- 

 tube of decomposed bouillon be poured out upon a large 

 flat surface, the individual bacteria in the mass are very 

 much more widely separated the one from the other 

 than they were when the bouillon was in the tube; but 

 they are in a fluid medium, and there is no possibility 

 of their either remaining separated or of their forming 

 colonies under these conditions, so that it is impossible 

 by this means to pick out the individuals from the 

 mixture. 



If, however, it is possible to discover some substance 

 which possesses the property of being at one time fluid 



