74 BACTERIOLOGY. 



of dilution, consisting of transferring a small portion of 

 the original mixture into a second tube of sterilized 

 bouillon to which gelatin had been added and liquefied ; 

 from this a similar portion was added to a third gelatin- 

 bouillon tube, and so on. These were then poured upon 

 large surfaces and allowed to solidify. The results were 

 entirely satisfactory. On the gelatin plates from the 

 original tube, as was expected, the colonies were too 

 numerous to be of any use ; on the plates made from 

 the first dilution they were much fewer in number, but 

 still they were usually too numerous and too closely 

 packed to permit of characteristic growth ; but on the 

 second dilution they were, as a rule, fewer in number 

 and widely separated, so that the individuals of each 

 species were in no way prevented by the proximity of 

 their neighbors from growing each in its own typical way. 

 There was then no difficulty in picking out the colonies 

 resulting from the growth of the different individual 

 bacteria. 



This, then, is the principle underlying Koch's method 

 for the isolation of bacteria by means of solid media, 



The fundamental part of the media employed is the 

 bouillon, which contains all the elements necessary for 

 the nutrition of most bacteria, the gelatin being em- 

 ployed simply for the purpose of rendering the bouillon 

 solid. The medium on which the organisms are 

 growing is, therefore, simply solidified bouillon, or beef 

 tea. 



In practice, two forms of gelatin are employed the 

 one an animal or bone gelatin, the ordinary table gelatin 

 of good quality ; and the other a vegetable gelatin, 

 known as agar-agar, or Japanese gelatin, which is ob- 

 tained from a group of algae growing in the sea along 



