134 BACTERIOLOGY. 



be overcome by allowing the rolled tubes to remain 

 in a nearly horizontal position for twenty-four hours 

 after rolling them, the mouth of the tube being about 

 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. <?., the 

 bottom of the tubes. After this they may be retained 

 in the upright position without danger of the agar- 

 agar slipping down. 



In both the plates and tubes, 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 because 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 numbers that there will 

 be nothing to prevent the characteristic 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. 



THE SERIAL TUBE METHOD OF SEPARATION. 

 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. 



