24 MANUAL OF BACTERIOLOGY. 



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 carmine in a httle water, and with 

 the microscope to study the trembHng motion exhibited by these 

 particles of carmine. It will be noticed that, although the 

 particles oscillate, no progress in any direction is accomplished 

 unless there are currents in the fluid. Such currents might give 

 rise to the impression that certain bacteria possessed motihty 

 when they were, in fact, powerless to move of themselves. In 

 the hanging-drop the multiplication 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 demonstration 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 prepara- 

 tion is no longer required, the slide and cover-glass should be 

 dropped into a 5 per cent, carbohc 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 appHed to a cover-glass, and mounted hke a 

 hanging-drop. The bacteria are kept in a layer close to the glass, 

 where growth may be studied. 



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



