ESMARGH TUBES. 123 



a horizontal position, the cotton end of the tube about 

 1 cm. higher than the bottom of the tube, for twenty- 

 four hours after rolling them. During 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 

 bottom 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.* 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 be- 

 cause of the great number of colonies 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 noth- 

 ing 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 



I The impression that agar-agar is not suitable for rolled tabes was shown 

 to be erroneous, and the above method was developed in the Pathological 

 Laboratory of the Johns Hopkins University. 



