MATERIALS. 
313 
count directly, count the number in one of the areas in the counting plate 
and multiply by the number of such areas covered by the whole plate. 
This will give the number of colonies on the plate, and this multiplied by 
1000, will give the number in the original milk. There may be any where 
from a few thousand to several millions. It frequently happens that the 
dilution of 1000 is insufficient, giving too many colonies on a plate. Higher 
dilutions are needed in such instances, but whether to make a higher dilu- 
tion can only be determined by a knowledge of the age and 
temperature of the milk and by experience. 
From these and from the plates of the following two ex- 
periments isolate and purify cultures as directed in No. 7. 
No. 5. Bacteria in the Air. Pour the contents of several 
tubes of agar into Petri dishes, replace the cover and allow 
to harden. Remove the cover from one of them and allow 
it to remain open in the laboratory for two minutes. Then 
replace the cover and place in the incubating oven. Expose 
a second in the same way in a barn; a third out of doors; a 
fourth in a barn after hay has been thrown down in front of 
cattle. .Other Petri dishes may be exposed in other locali- 
ties. After twenty-four hours' incubation count the num- 
ber of colonies and compare the relative number of bacteria 
in the air at the different places. 
No. 6. Bacteria on the Fingers. Pour a Petri dish as 
above directed and allow to harden. Remove the cover of 
one and touch it gently with the fingers in several places. 
The hands may then be thoroughly washed, and a second 
Petri dish treated in the same way. Incubate for twenty- 
four hours and see if bacteria colonies have grown where the 
fingers touched the agar. 
No. 7. Isolation and Purification of Bacteria. Any of 
the plates above prepared will show after proper incubation 
a number of colonies. Comparing the different colonies 
noticeable differences will be seen between them. These 
differences in the colonies commonly indicate different kinds 
of bacteria, for the same kind of bacteria produce, com- 
monly, the same kinds of colonies. 
Isolation. Sterilize a platinum needle (Fig. 62), in a flame until red-hot 
and after allowing it a few seconds to cool, dip the tip of it into one of the 
colonies. Transfer the bacteria adherent to the needle to one of the agar 
slant tubes by removing the plug and drawing the tip of the wire over 
the surface of the agar. Sterilize the needle again before laying it down. 
Label the tube with a gum label telling its source. In a note-book make a 
brief record of the kind of colony from which it was obtained. Allow the 
inoculated tube to grow, either in the incubating oven or in the room, until 
a growth appears on the surface of the agar. This is an agar slant. 
Purification. If the colony thus isolated has grown from a single bac- 
terium this growth on the agar will be a pure culture. But this is not 
27 
FIG. 62. 
Platinum 
needle and 
platinum loop. 
