306 PRACTICAL COURSE IN BOTANY 
of all that is foul and loathesome, are engaged in an unceas- 
ing work as scavengers, without which life would no longer 
be possible on our globe, as will be shown in the following 
section. 
A. BAcTERIA 
Materia. — A vessel of water in which hay has been left to soak for 
several hours; a freshly boiled potato. 
Appiiances. — A double boiler for sterilizing; a number of clean glass 
jars and bottles; cotton wool for stoppers; a compound microscope. 
Cutturre Mepiums.—A freshly boiled potato answers very well for 
ordinary purposes. ‘‘Bread mash’ can be made by drying some bread 
crumbs in an oven, then mashing and mixing them to a paste with boiling 
water ; sterilize by three successive heatings in a double boiler. A sterilized 
preparation of gelatine solution is the medium most commonly used. 
347. How to obtain specimens for observation. — While 
bacteria are plentiful almost everywhere, it is not always 
easy to capture them just when and where you want them. 
For this purpose, put some hay in water and leave in a 
warm place away from the light until the liquid becomes 
cloudy or a film forms on the surface. This will show that 
. bacteria are present. If it is desired to study any particu- 
lar kind of bacterium, inoculate one of the culture mediums 
described under ‘‘ material,’’ or a few drops of sterilized 
extract of beef, with a small quantity of the substance to be 
examined, or with dust or scrapings from the locality under 
consideration. 
EXPERIMENT 93. By WHAT MEANS ARE BACTERIA COMMONLY DISTRIB- 
urep ? — Put a slice of freshly boiled potato into each of three glass tum- 
blers and cover with a filter of cotton wool held in place by tying tightly 
with a cord, or by an elastic band. Set them all in a vessel of water, bring 
it to a boil, and keep at that temperature for half an hour, to sterilize the 
air in the tumblers. When they have cooled, lift the cotton from (1) for 
a minute or two and then replace. Carefully pass the tip of a medicine 
dropper through the filter of (2) so as to prevent the entrance of unster- 
ilized air, and put on the slice of potato a small quantity of the bacterial 
liquid prepared as directed in the last paragraph. Leave (3) unopened. 
Keep all together in a warm, dark place and observe at intervals of from 
12 to 24 hours. Do any bacteria appear in (3)? Do any appear on the 
