48 BACTERIOLOGICAL DIAGNOSIS. 
thionin on to the film and allow it to act for 2 minutes, 
Wash under the tap. 
Dry by pressing carefully with strips of blotting-paper 
and then in the flame. Place a drop of balsam upon 
the film and apply a cover-glass. 
Prepare a second film and stain by Gram’s method 
(Pp. 32). 
The films are now examined microscopically (see 
p- 34). We shall defer the description of the points 
upon which a diagnosis is to be based until we deal 
with the examination of cultures. 
CutturaL MeEtTuHops. 
The diphtheria bacillus grows best at or near the 
body temperature (about 37° C.) and flourishes on 
almost all culture media. But agar is scarcely ever 
used in growing it for diagnostic purposes: this 
medium -serves well for the cultivation of a great 
many organisms, some of which are almost always pre- 
sent in the mouth, so that cultures made upon it are 
usually very impure. We use a medium which permits 
the development of the diphtheria bacillus and inhibits 
that of most other organisms. The best are ascitic 
and solidified blood-serum. 
.. The method in which the medium is ientuiee is as 
fellows :—The tube of culture medium and the tube 
containing the swab are held side by side between the 
index and middle fingers of the left hand, the mouths 
of the tubes pointing to the right and slightly down- 
wards. The-plug of the culture tube is then singed, 
removed by means of a pair of forceps, and placed be- 
tween the ring and little fingers of the left hand. The 
