192 LABORATORY EXERCISES IN BACTERIOLOGY. 



Smears for Stained Preparations. The film may be spread upon either a 

 clean slide or cover-slip, the former being much preferable in that it is more conveniently 

 handled and is in less danger of being broken. The writer usually places the film a 

 little to one side of the center of the slide (not so far as to interfere with centering on the 

 mechanical stage), and is thus relieved of the necessity of using a forceps for handling 

 the preparation. When made on a cover it is essential to use one of the different forms 

 of forceps (cover-glass holder), as that of Stewart (Fig. 58), in order to pursue the work 

 without danger to the preparation or of getting the germs upon the fingers. 



The surface of the slide or cover must be entirely clean and quite free from traces 

 of grease which are apt to come on the glass from handling. When taken from the 

 ammoniated alcohol in the tray the slide or cover should be carefully dried with an 

 old linen rag which is quite free from grease from inferior soap or other source, or with 

 a bit of clean, unsized, unfilled paper. A drop of water on the surface should spread 

 out flat and evenly and not separate into small droplets or persist in globular form 

 when it is attempted to draw it over the surface with the needle. Should the drop 

 refuse to spread readily it may be concluded that the glass is not entirely clean; and 

 in such case it is best to wash it with good soap and water, and then rinse it first in 

 water and then in alcohol, and again dry it with paper or a clean rag. 



(a) In examination of cultures it is preferable to obtain the germs from a growth 

 on a solid medium, the bouillon in a film rendering uniform staining difficult. A small 



FIG. 58. STEWART FORCEPS FOR COVER-GLASSES. 



drop of clean water is placed in proper position on a slide or cover. The platinum 

 needle, having been flamed, is used to obtain a trace of the culture to be examined, 

 which is then diffused in the drop until a very faint turbidity is noticed (best seen in 

 a side light) . With the needle the water with its contained bacteria is now spread over 

 a convenient area, the needle at once flamed to destroy the infection remaining on it 

 and the film allowed to dry in the air. (If a cover was used the film might have been 

 spread by application of a second, clean cover, the two after close adjustment being 

 slipped apart so as to evenly distribute the fluid over the adjacent surfaces of the two 

 covers. In this case use forceps in handling.) The drying is completed and the film 

 fixed to the surface of the slide by passing the latter by a slow, even movement through 

 the flame of a Bunsen burner or spirit lamp several times, the film on the upper side 

 and away from the flame, to prevent scorching. Usually, three passages through the 

 flame will suffice ; the glass should be distinctly hot, but not sufficiently to be actually 

 painful when touched to the back of the hand. Thus prepared the film should be 

 visible as a delicate opacity, with dead, non-glistening surface. It is now ready for 

 staining or may be kept indefinitely (placed with film upward on a sheet of paper 

 bearing proper label and covered over with suitable cover to keep the atmospheric 

 dust from settling on it or in a small paper box). 



(6) Smears of Organic Fluids Containing Bacteria. Fluids containing organic 

 matter in which bacteria are diffused are made into similar smear preparations by 



