140 BACTERIOLOGY. 



1 cm. higher than the bottom. During this time the 

 margin of the agar-agar nearest the cotton plug dries 

 and becomes adherent to the walls of the tube, while 

 the water collects at the most dependent point — i. e., the 

 bottom of the tubes. After this they may be retained 

 in the upright position without danger of the agar- 

 agar slipping down. In all these manipulations, 

 if the dilutions of the number of organisms have been 

 properly conducted, the results will be the same. The 

 original plate or tube, as a rule, will be of no use be- 

 cause of the great number of colonies in it ; plate or 

 tube No. 2 may be of service ; but plate or tube No. 3 

 will usually contain the organisms in such small num- 

 bers that there will be nothing to prevent the charac- 

 teristic development of the colonies originating from 

 them. 



For reasons of economy the "original," tube No. 1, is 

 sometimes substituted by a tube containing normal salt- 

 solution (0.6 to 0.7 per cent, of sodium chloride in 

 water), which is thrown aside as soon as the dilutions 

 are completed, and only plates or tubes Nos. 2 and 3 

 are made. 



Another method for the separation of bacteria and 

 their isolation as single colonies consists in the making 

 of dilutions upon the surface of solid media, such as 

 potato, coagulated blood-serum, agar-agar, and gelatin. 

 In pursuance of this method one selects a number of 

 tubes containing the medium set in a slanting position. 

 With a platinum needle a bit of the substance to be 

 studied is smeared upon tube No. 1 ; without sterilizing 

 the needle it is passed in succession over the surface 

 of the medium in tubes Nos. 2, 3, 4, etc. When de- 

 velopment has occurred essentially the same conditions 



