224 BACTERIOLOGY. 
this would cause the fibres of the cotton plugs to adhere 
to the sides of the tubes when the media dried and 
make it difficult to remove the plugs wholly when we 
wished to inoculate the contents of the tubes. 
The tubes and flasks, plugged with sterile cotton 
and full of media, are put in the steam sterilizer for 
one half hour on three consecutive days, or in the 
autoclave for twenty minutes for two consecutive days. 
A portion of the tubes containing nutrient agar are 
laid in a slanted position before cooling, after the final 
sterilization, so that a larger surface may be obtained. 
Technique of Making Plate Cultures. 
When we make cultures from any material, we are 
very apt to find that instead of one variety of bacteria 
only there are a number present. If such material 
is placed in fluid media contained in test-tubes, we find 
that the different varieties all grow together and be- 
come hopelessly mixed. When, on the other hand, the 
bacteria are placed on solid media they develop about the 
spot where they were inoculated. If different varieties, 
however, are placed too near together, they overgrow 
one another ; it is thus advisable to have a greater sur- 
face of nutrient material than is given on the slanted 
surface of nutrient agar or blood-serum contained in 
test-tubes. This need is met by pouring the media 
while warm on flat, cool, glass plates or into shallow 
dishes. In making plate cultures two methods are 
carried out. In the first the material with its contained 
bacteria is scattered throughout the fluid before it 
hardens; in the second it is streaked over the surface of 
the medium after it has solidified. Nutrient agar and 
nutrient gelatin, the two substances used for plate 
