4 
North Carolina Agricultural Experiment Station 
anticipated, that the jellifying power is modified by the proportion of colloidal 
material added. The results with 1 and 2 per cent agar are shown in Table 1. 
It will be noted that the limits of solidification of 1 per cent agar are ap¬ 
proximately 5.11 per cent acid and 0.39 per cent alkali, whereas 2 per cent 
agar does not lose its jellifying power until 6.51 per cent acid or 0.58 per cent 
alkali has been added. A better appreciation of the degree of acidity and alka¬ 
linity of these limits can be gained when they are compared with pH values 
determined by the colorimetric method of Clark and Lubs (1917). It was 
found that the addition of 0.1 cc. of HC1 to 10 cc. of agar gave a concentra¬ 
tion of about pH 1.4 and 0.03 cc. NaOH a concentration of about pH 9.2. In 
reaching the limits in 2 per cent agar it will he seen that 20 times this volume 
of acid and 9 times this volume of alkali were employed. Manifestly, these 
limits are only approximate and could be more accurately determined by im¬ 
proved technic. They are, however, far beyond the limits of tolerance of 
microorganisms and are intended only to show that hydrogen or hydroxyl ion 
concentration need not he limiting factors in the preparation of solid agar 
media. 
Table 1.—Effect of Acid and Alkali on Solidification of Agar 
Agar 
cc. 
10 
10 
10 
10 
10 
10 
10 
10 
10 
10 
1 Per Cent Agar 
2 Per Cent Agar 
HCl 
NaOH 
Physical 
State, 
20° C. 
Agar 
HCl 
NaOPI 
Physical 
State, 
20° C. 
Per 
Per 
Per 
Per 
cc. 
Cent 
cc. 
Cent 
cc. 
cc. 
Cent 
cc. 
Cent 
10 
2.0 
6.51 
Semisolid 
10 
1.7 
5.68 
Solid 
1.5 
5.11 
Semisolid 
10 
1.5 
5.11 
Solid 
1.2 
4. 19 
Solid 
10 
1.2 
4.19 
Solid 
1.0 
3.55 
Solid 
10 
1.0 
3.55 
Solid 
0.7 
2.55 
Solid 
10 
0.7 
2.55 
Solid 
0.5 
1.86 
Solid 
10 
0.5 
1.86 
Solid 
0.3 
1.14 
Solid 
10 
0.3 
1.14 
Solid 
0.1 
0.38 
Solid 
10 
0.1 
0.38 
Solid 
0.1 
0.19 
Solid 
10 
0.1 
0.19 
Solid 
0.2 
0.39 
Semisolid 
10 
0.2 
0.39 
Solid 
0.3 
0.58 
Liquid 
10 
0.3 
0.58 
Semisolid 
All of the agar media in these series appear to be able to remain solid for 
an indefinite period when they are maintained at room temperature. If, how¬ 
ever, they are autoclaved and then cooled all of the acid media were found to 
remain liquid, whereas the tubes of 1 per cent agar with 0.1 cc. NaOH and 
2 per cent agar with 0.1 and 0.2 cc. NaOH became solid again. 
Considerably larger amounts of acid or alkali must be added to 10 and 15 
per cent gelatin to destroy the jellifying power as shown in Table 2. 
In the case of 10 per cent gelatin the limits are seen to he about 9.02 per 
cent HC1 and between 3.33 and 4 per cent NaOH, and of 15 per cent gelatin 
between 9.02 and 10.15 per cent HC1 and about 4 per cent NaOH. When these 
media were examined after having been maintained in an ice chest at about 
10°C. for twelve hours, all of those to which more than 1 cc. of NaOH had been 
added were found to have become liquid, and a heavy whitish precipitate had 
formed. All of the tubes to which HCl had been added were still solid, how- 
