W. O. Fenn 441 
1. Salts with monovalent ions (like NaCl) decrease the effect 
of both acids and alkalies in gelatin. 
2. Salts with bivalent (like CaCl.) or trivalent cations. de- 
crease the effect of alkalies on gelatin but increase the effect of 
acid, except in high concentrations of salt or acid, where the 
effect is decreased. : 
3. Salts like Na,SO, with bivalent or trivalent anions de- 
crease the effect of acids on gelatin but increase the effect of 
alkalies, except in high concentrations, when the greater the con- 
Cc. ALC.. TO PPT: 
GEL- IN HeO CONTROL 
7 
15 ee GEL. IN NaCl 
IN NaNOs 
IN Na—Ac 
10 : 
i” | : IN Na:-Cit. 
Vs =: : | 
1 Sars. IN NazSOs 
5 / 
0 | 5 1M 
ACETIC ACID——> 
Fiag. 1. Curves showing the effect on’ the alcohol number (ordinates) of 
increasing concentrations of acetic acid (abscisse) in gelatin dissolved in 
water (control) and various sodium salts, all 0.125 m. The sodium salts 
antagonize the effect of the acetic acid and make precipitation by alcoho! 
possible even in strong acetic acid. (See Table I.) 
centration of either salt or alkali, the less the alcohol necessary 
for precipitation. | 
The effect of various sodium salts on gelatin plus acetic acid is 
illustrated in Fig. 1 and Table I. Acetic acid alone entirely 
prevents the precipitation of gelatin by alcohol except in com- 
paratively low concentrations. In the presence of sodium salts, 
0.125 mM, however, precipitation by alcohol is possible even in 
