26 STAINING METHODS. 
suspended in an albuminous medium it will be necessary, after the 
film is dry, to heat the preparation sufficiently to coagulate the albu- 
men, in order that it may not be washed off in the subsequent stain- 
ing process. This is best done, in accordance with Koch’s directions 
for the preparation of tuberculous sputum, by passing the cover 
glass, held in slender forceps, rather quickly through the flame of an 
alcohol lamp three times in succession. In this operation it must 
be remembered that too much heat will destroy the preparation, 
while too little will fail to accomplish the object in view—coagu- 
lation of the albumen. In passing the cover glass through the 
flame the smeared side is to be held upward. The time required 
will be about three seconds for passing it three times as directed ; 
but this will vary according to the intensity of the flame, and some 
little experience is necessary in order to obtain the best results. 
The operation of ‘‘ fixing,” or coagulating the albumen, may also 
be effected by exposure in a dry-air oven, heated to 120° to 130° C., 
for a few minutes (two to ten minutes), as directed by Ehrlich. 
Bacteria simply suspended in distilled water adhere very well to 
the cover glass when treated as directed, but if they have been taken 
from a liquefied gelatin culture the film is very apt to be washed 
away during the staining process. This is best avoided by taking as 
little as possible of the gelatin medium and suspending the bacteria 
to be examined in a drop of water, which dilutes the gelatin and 
washes it away from the surface of the cells, 
Smear Preparations.—In various infectious diseases bacteria are 
found in the blood and tissues of the body, and their presence may 
be demonstrated by making what is called a smear preparation. A 
little drop of blood may be spread upon the thin glass cover, or it 
may be brought in contact with the freshly cut surface of one of the 
vascular organs, as the liver or spleen. It is especially desirable that 
the material used for such a preparation be small in amount and dis- 
tributed evenly in a very thin layer. In Germany it is the custom, 
in making smear preparations, to press the material between two glass 
covers, which are then separated by sliding them apart, thus leaving 
a thin layer upon each. This answers very well, but the writer pre- 
fers to spread the material by drawing across the face of the cover 
glass the end of a well-ground and polished glass slide. This method 
is especially useful for spreading blood in a uniform layer, in which 
the corpuscles are evenly distributed and retain their normal form. 
A very small drop of blood is placed near one edge of the cover glass, 
which is placed upcn a smooth surface ; the glass slide is held at a 
very acute angle and is gently drawn across the cover glass, without 
any pressure. 
Most bacteriologists make their preparations upon the cover glass, 
