ESMARCH TUBES. 149 



horizontal position and twisted between the fingers 

 until the sides are moistened by the contents to within 

 about 1 cm. of the cotton plug, care being taken that 

 the gelatin does not touch the cotton ; otherwise the latter 

 becomes adherent to the sides of the tube and is difficult 

 to remove. The tube is then placed in the groove in 

 the ice and rolled, neither rubber cap nor cutting off of 

 the cotton plug being necessary. 



The advantages of this process over that followed by 

 v. Esmarch are that it requires less time, is cleaner, 

 no rubber caps are needed, the rolled tubes are more 

 uniform, and the gelatin does not touch the cotton plug, 

 as is always the case in tubes rolled under water, be- 

 cause of the impossibility of keeping them at one level. 



There is an erroneous impression that Esmarch tubes 

 are not a success when made from ordinary nutrient 

 agar-agar because of the tendency of this medium to 

 shrink and slip to the bottom of the tube. This slip- 

 ping clown of the agar-agar is due to the water, which is 

 squeezed from it during solidification, getting between 

 the medium and the walls of the tube. This can easily 

 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 all these manipulations, 

 if the dilutions of the number of organisms have been 

 properly conducted, the results will be the same. The 



