198 BACTERIOLOGY. 
misleading results. After death we have an added dif- 
ficulty, in that even the blood and body tissues become 
invaded by bacteria from the intestines and elsewhere, 
so that bacteria actually present in the diseased tissues 
may have had no connection with the disease under in- 
vestigation. Whenever bacteria are found, therefore, 
the methods carried out in the investigation should be 
most carefully examined, to see if some error in tech- 
nique has not been committed. The aim of the 
bacteriological examination of any material is to de- 
termine whether bacteria are present or not, and, if 
present, to ascertain their number and distribution, and, 
if possible, their species. This is accomplished chiefly 
by means of two methods—viz., the direct examina- 
tion with the microscope of cover glass preparations 
and the results of cultures made from the material. 
Sometimes animal inoculations are also employed. 
The direct microscopical examination of suspected sub- 
stances for bacteria can be made either with or without 
staining. Unstained, the bacteria are examined, to note 
their motility, their form,and their general arrangement: 
but for more exact study, they can be so much better 
observed when stained that this step is always advisable. 
A cover-glass preparation is made as follows: A 
very small amount of the blood, pus, discharges from 
mucous membranes, culture fluids, or other material to 
be examined is removed by means of a sterile swab or 
platinum loop and smeared undiluted in a thin film 
over a clean, thin cover-glass.’ From cultures on solid 
1 To render new cover-slips clean and free from grease, place them in strong 
nitric acid for a few hours, then rinse them off in water, then in alcohol, 
then in ether. Place them finally for keeping in alcohol, to which a little 
ammonia has been added. When used wipe with soft clean handkerchief. 
If old cover-slips are used boil first in soda solution. 
