24 MANUAL OF BACTERIOLOGY. 



low power and then to focus upon this edge with the oil-im- 

 mersion objective. Above all things guard against breaking 

 the cover-glass by forcing the objective down upon it. The 

 motility of certain bacteria is one of the most striking phenom- 

 ena to be observed in the hanging-drop. It is not to be con- 

 fused with the so-called "Brownian movement" which is ex- 

 hibited by fine particles suspended in a watery fluid. It is well 

 for the beginner to observe the character of the Brownian move- 

 ment by rubbing up some dry, powdered carmine in a little 

 water, and with the microscope to study the trembling motion 

 exhibited by these particles of carmine. It will be noticed 

 that, although the particles oscillate, no progress in any direc- 

 tion is accomplished unless there are currents in the fluid. 

 Such currents might give rise to the impression that certain 

 bacteria possessed motility when they were, in fact, powerless 

 to move of themselves. In the hanging-drop the multiplica- 

 tion of bacteria can be studied, the formation of spores and the 

 development of spores into fully formed bacteria. The 

 hanging-drop has recently been put into service for the demon- 

 stration of the so-called serum-reaction with the bacillus of 

 typhoid fever. Sometimes bacteria must be watched in the 

 hanging-drop for hours, or even days, and it may be necessary 

 to keep it at the temperature of the human body for this length 

 of time. Various complicated kinds of apparatus have been 

 devised for this purpose, but they are needful only with special 

 kinds of work. When the hanging-drop preparation is no 

 longer required, the slide and cover-glass should be dropped 

 into a 5 per cent, carbolic acid solution and afterward sterilized 

 by steam. 



Hanging block preparations, which were introduced by 

 Hill,* consist in the use of a cube of nutrient agar instead of a 

 drop of fluid. Bacteria are distributed on the surface of the 

 agar, which is then applied to a cover-glass, and mounted like 



* Journal of Medical Research. Vol. VII. March, 1902. 



