COLLECTION OF PUS. 137 
such circumstances unless the surface of the medium 
has previously been coated with blood. 
Another method is to make gelatin plates. This isa 
very simple matter if the materials are at hand. 
Requisites.-—1. Two or three tubes of gelatin. 
2. Two or three sterilised Petri’s dishes. 
3. A platinum needle—a loop will be best. 
Process.—Inoculate a gelatin tube in the manner de- 
scribed on p. 22, and then melt it by immersion in 
warm (not hot) water. 
Distribute the pus throughout the melted gelatin by 
rolling the tube between the hands, and by tilting it 
from side to side. Do not shake, and do not let the 
melted gelatin touch the cotton-wool plug. 
Take a loopful of the gelatin and transfer it to a 
second culture tube. Melt the gelatin in this and mix 
as before. Proceed to inoculate a third tube from the 
second one if you think it probable that the pus is very 
rich in organisms. 
Now take the first tube and singe the projecting part 
of the wool plug and heat the mouth of the tube in 
order to destroy any germs which may be upon it; 
allow it to cool. 
Place the Petri dish on the table in front of you and 
raise the lid sufficiently to allow you to insert the end of 
the test-tube; do this, and tilt the latter so that the 
melted gelatin flows into the dish. Immediately replace 
the lid, and tilt and roll the dish until the gelatin forms 
an even film over its whole lower surface. Place it on 
a flat table to set. Repeat the process with the other 
tubes. Incubate at about 20° C. for two or three days. 
Examine the dishes, placing them on the stage of the 
microscope and using the low power. Each organism 
will have grown into a small colony, which will resemble 
