BACTERIOLOGICAL TECHNIQUE. 225 
cultures, differ in two essential points, which cause 
some difference in their uses. Nutrient 1 per cent. 
agar melts at a high temperature and begins to thicken 
at about 36° C. ‘It is not liquefied by bacterial fer- 
ments. Nutrient 10 per cent. gelatin melts at the low 
temperature of about 23° C. and solidifies at a point 
slightly below that. It is liquefied by many bacterial 
ferments. When we wish to inoculate fluid nutrient 
agar for plate cultures we have to take great care that 
in cooling it to a point which will not injure the bac- 
teria, about 41° C., we do not allow it to cool too much 
and thus solidify and prevent our pouring it into the 
plates. To prevent this, when a number of tubes are 
to be inoculated they are placed while still hot in a 
basin of water which has been heated to about 45° C. 
When the temperature of the agar in one of the tubes, 
Petri dish. 
as tested by a thermometer, has fallen to 40° the water, 
milk, feces, bacterial culture, or other substances to he 
tested are added to the other tubes in whatever quantity 
is thought to be proper. After inoculation the con- 
tents of the tubes are thoroughly shaken and poured 
out quickly into round, flat-bottomed glass dishes (Fig. 
27), the covers of which are removed for the required 
time only. The bacteria are now scattered throughout 
the fluid, and as it quickly solidifies they are fixed 
wherever they happen to be, and thus as each individual 
15 
