THE EXAMINATION OF CULTURES 19 
29. Making hanging-drop preparations. (a) From a bou- 
illon culture. Place a clean cover glass on a tray. With the 
loop remove a drop of the liquid culture and place it on the 
middle of the cover glass. With a pair of fine forceps invert 
the cover glass over the glass ring fixed to a slide for this 
purpose. The surface of the ring should previously be mois- 
tened with liquid vaseline to prevent the cover glass from slid- 
ing. ‘The preparation is then ready for examination. Examine 
it first with the high-power dry lens and then with the oil- 
immersion objective. (For directions on the use of the 
microscope, see Zhe Microscope, by Professor S. H. Gage.) 
(0) From cultures on solid media. On account of the very 
large number of bacteria in the growth on solid media it is 
necessary to separate them in a clear liquid. Take a cover 
glass as before and place a loopful of bouillon or sterilized 
water on the center. ‘Touch the surface growth very gently 
with the end of the platinum needle and carefully rinse it in 
the drop of liquid on the cover glass. From this point the 
examination is the same as with the liquid culture. Upon 
examination, ‘if the bacteria are so numerous that the indi- 
vidual organisms cannot be clearly distinguished, i.e. separated 
from each other, the preparation must be rejected and another 
one made, using a smaller quantity of the growth. After 
examination the cover glasses should be placed at once in a 
glass jar containing a strong disinfectant (5% carbolic acid, 
1 to 1000 corrosive sublimate solution, or a strong solution 
of a mineral acid). 
30. Suggestions for the microscopic examination of living 
bacteria. In examining the bacteria as they appear under 
the microscope in the hanging-drop preparation the following 
features should be observed: Are the individual bacteria 
spherical, rod-shaped, or spiral in form? Are they single or 
united in pairs, masses, or clumps, or in shorter or longer 
chains? For this determination it is better to examine the 
organisms near the edge of the drop. Are they motile, that is, 
