THE PREPARATION OF GELATIN AND AGAR II 
EXERCISE IV 
THE PREPARATION OF GELATIN AND AGAR 
15. Of the solid media employed in cultivating bacteria, agar 
d gelatin are most commonly used. They depend for their 
\tritive properties largely upon the bouillon from which they 
e made, the agar and gelatin forming simply the solidifying 
ements. The striking difference between the two is that the 
latin melts at the body temperature, whereas the agar is not 
adily liquefied below the boiling point. For this reason 
latin is not used as a solid medium for cultivating bacteria 
a high (body) temperature. There are several processes for 
eparing these media, but the addition of the dry agar and 
Jatin to bouillon (§ 12) either immediately after it is filtered, 
later after it has been sterilized and stored in flasks, seems 
be the most convenient procedure. The agar itself is usu- 
y neutral in reaction, but the gelatin often has a decidedly 
id reaction. This necessitates the careful retesting of the 
action of the two media, even though neutral or slightly 
xaline bouillon is used. 
16. Work for this exercise. See that bouillon made in 
xercise ITI is properly sterilized. Prepare 300 cc. of gelatin 
d 300 cc. of agar, i.e. start with 300 cc. of bouillon for each. - 
iere will be considerable shrinkage owing to the amount lost 
. the dishes, filter, etc., so that the quantities of media will be 
ipreciably less than this amount. Distribute each medium as 
llows : 
Put 5 cc. in each of 10 small sterile test tubes. 
Put 10 cc. in each of 12 large sterile test tubes. 
Put the remainder in the flask which contained the bouillon. 
17. The preparation of nutrient gelatin. Take a flask of 
uillon containing 300 cc. and pour it into a small agate-iron 
