ESMARCH TUBES. 



139 



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. 



Fig. 29. 



Demonstrating Booker's method of rolling Esmarch tubes on a block of ice. 



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 down 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 



