PREPARATION OF NUTRIENT AGAR. 35 
occasional additions of small quantities of water until it is thoroughly cooked in 
the form of a thick mush. It is then put into the remainder of the water or bouil- 
lon and subjected to streaming steam for two hours, after which, if the first heating 
was sufficient, it filters readily without the use of a hot-water filter, or the necessity 
of keeping it in the steamer during the filtering. ‘The stirring rod must touch all 
parts of the bottom of the dish exposed to the flame, every few seconds during the 
preliminary heating, otherwise the agar will burn on and be spoiled. On some 
accounts it is best to begin 
operations with beakers rather 
than the enameled iron dishes. 
In this way all likelihood of 
using burned agar is avoided, 
since the moment the agar 
burns on the beaker cracks and 
the agar is spilled. For bacte- 
riological use agar should be 
clear, not cloudy or filled with 
unremoved precipitates. 
The writer now employs an 
autoclave and uses an agar flour 
procured from Lautenschlager or 
Merck (fig.33). If one has an au- 
toclave the preliminary heating 
of the agar in an open dish with 
a minimum quantity of water 
and all the subsequent stages 
may be dispensed with and the 
entire process carried on in the 
autoclave, unless it is known 
or suspected that media heated 
in the autoclave are less well 
adapted to the growth of par- 
ticular organisms than those pre- 
pared atroo° C. ‘The amount 
of agar added to the: culture 
fluid is usually r per cent. On 
the making of nutrient agar 
consult “Formulz,” and the various standard text-books. 
Is there any difference in the appearance of colonies when grown at 5° to 10°, 
15° to 20°, and 30° to 37° C.? Observe the amount of precipitate that collects in 
the fluid in the V. For other observations as to growth on this substratum see 
“Gelatin.” Every organism should be studied in numerous Petri-dish poured-plate 
Fig. 33.* 
*F ic, 33.—Agar-agar flour as received from European manufacturers. Package of Merck’s 
agar flour. 
