no . BACTERIOLOGY. 



zontal position, the cotton end of the tube about 1.5 to 

 2 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 

 slipping 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 because 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 nothing to prevent their characteristic de- 

 velopment. 



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 dilu- 

 tions are completed and only plates or tubes 2 and 3 

 are made. 



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 

 Laboratory of the Johns Hopkins University. 



