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, 



FIG. 27. 



Petri dish. 



as tested by a thermometer, has fallen to 40 the water, 

 milk, feces, bacterial culture, or other substances to be 

 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 



