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 



