56 MICRO-ORGANISMS AND DISEASE [chap. 



contains now probably only one kind of organism. To 

 make it quite certain, inoculate from this a new culture- 

 tube in the same manner, and now you probably have 

 sown only a single species. In this manner by continued 

 transference it is possible to obtain cultures of only one 

 species of organisms. Many conditions, such as naked-eye 

 appearances of a particular kind, coloration of the culture- 

 medium, formation of a pellicle, the quantity of growth in a 

 given time, soon indicate whether we have the desired single 

 species ; in some instances it is, however, extremely difficult 

 to isolate after this method. 



The method of " dilution " means diluting the material 

 containing the mixture of the various species to a very 

 large extent with some sterile indifferent fluid, such as well- 

 boiled sahne solution of o'6 per cent., and then inoculating 

 new tubes with a droplet of this greatly diluted material. 

 For this purpose take up with a platinum needle or loop a 

 droplet of the mixture, then transfer it in to a test-tube or flask 

 containing well-boiled saline solution, so as to greatly dilute 

 (looo-fold or more) the particle or droplet of old culture- 

 material, and from this dilution inoculate then a series of new 

 culture-tubes containing different nourishing material, using 

 always only a trace for inoculation. In this way it is prob- 

 able that, owing to the great dilution, the trace of a droplet 

 of this mixture used for the new inoculation contains only 

 one species. Using a series of new culture-tubes and inocu- 

 lating them thus, after twenty-four hours of incubation it 

 will be found that some tubes have not received any seed, 

 others only one species. If it be required to dilute the 

 original fluid greatly, say if it teems with and is turbid by 

 different organisms, then a droplet of this is placed into a 

 large flask containing the well-boiled saline solution, so 

 that a dilution of i in t, 000,000 or more can be effected. 



