228 BACTERIOLOGY. 
The nutrient agar-agar is frequently used in a dif- 
ferent manner. A small quantity is poured into the 
Petri dish and allowed to harden. The substance to 
be tested bacteriologically, or a dilution of it, is then 
streaked by means of a platinum loop lightly over its 
surface. While in the former method most of the bac- 
~ teria developed under the surface, here all develop upon 
it. This is an advantage, as many forms of bacteria 
develop more characteristically on the surface than in 
the midst of the media, and it is easier to remove them 
free from other bacteria with the platinum needle. The 
method of using glass plates upon a cooling stage has 
now been practically given up for the more convenient 
one of Petri dishes. In warm weather the dishes should 
be cooled before using, so as to harden quickly the agar 
or gelatin that is poured into them. 
An old method, which is still sometimes used to find 
the number of living bacteria, is, instead of pouring 
out the media which has been inoculated, to congeal it 
on the sides of the test-tube. This is best done by 
laying the tube flat on its side on a cake of ice and 
rotating it. Tubes come especially formed for this by 
having a slight neck, which prevents the media run- 
ning up to the plugged end of the tube. This method, 
Esmarch’s, is used only when the Petri dishes are not 
obtainable or cannot easily be transported. 
The Study of Colonies in Plate Cultures in Nutrient Agar. 
The plates should be removed after twelve to twenty- 
four hours’ growth at blood temperature and after one 
to three days at 70°. The special time allowed varies 
according to the rapidity of the growth of the varieties 
developing, thus bacteria, such as the streptococci and 
