TUBES 143 



rolled tubes to remain in a nearly horizontal position for 

 twenty-four hours after rolling them, the mouth of the tube 

 being about 1 em. 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 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. With a platinum needle a bit of the sub- 

 stance 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 develop- 



