118 BACTERIOLOGY. 



four hours after rolling them. Daring this time the 

 edge of the agar-agar nearest the cotton plug becomes 

 dried and adherent to the walls of the tube, while the 

 water collects at the most dependent point, i. e., the bot- 

 tom of the tube. After this they may be retained in 

 the upright position without fear of the agar-agar slip- 

 ping down. We have followed this process for several 

 years with entire satisfaction. 1 



In all these processes, 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 col- 

 onies in it. Plate or tube No. 2 may be of service, but 

 plate or tube 3 will usually contain the organisms in 

 such small numbers that the colonies originating from 

 them will have nothing to prevent their characteristic 

 development. 



For reasons of economy, the "original," tube 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 '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. 

 For the performance of this method one selects a num- 

 ber of tubes containing the medium to be employed in 

 a slanting position. With the platinum needle a bit of 

 the substance to be studied is smeared upon tube No. 1 ; 



1 The impression that agar-agar is not suitable for roll tubes was shown to 

 be erroneous, and the above method was developed in the Pathological Lab- 

 oratory of the Johns Hopkins University. 



