536 
PHYSIOLOGY: W. O. FENN 
offers greatest resistance to the dehydration effect of the alcohol, by 
which precipitation is ultimately accomplished. 
If now salt A be gradually replaced by an antagonistic salt B, less 
and less alcohol is required for precipitation until a minimum is reached. 
This minimum is at the isoelectric point, corresponding to greatest 
aggregation. At this point, if the concentration of the salts he sufficiently 
high the gelatine is precipitated without alcohol. If all of salt A be re- 
placed by salt B, the amount of alcohol needed for precipitation again 
rises to the point characteristic of that salt in the concentration used. 
Thus an antagonism curve can be plotted, such as that shown in figure 
2 for CaCl2 and sodium citrate and in figure 3 for NaCl and CaCl2. 
The probable additive effect in each figure is represented by the dotted 
base line. Such lines at least have resulted from similarly arranged 
experiments with non-antagonistic salts such as NaCl vs. KSCN and 
CaCl2 vs. MnCl2.« 
It is frequently more convenient in determining antagonism to keep 
the concentration of one salt constant and gradually increase that of 
the other. If the two salts are additive to each other, the alcohol needed 
for precipitation is thereby increased. Thus when CaCl2 is added to 
gelatine containing HCl, or when Na2S04 is added to gelatine containing 
NaOH, the amount of alcohol needed for precipitation is increased.^ 
If two antagonistic salts are used, on the other hand, there is a decrease 
in the amount of alcohol needed for precipitation until a minimum is 
reached. This is again an isoelectric point and here the gelatine may 
be precipitated without alcohol if the salt concentration be sufficiently 
high. If still more of the salt be added, there is an increase of the amount 
of alcohol needed for precipitation and a decrease of precipitability 
by salts in the absence of alcohol. Such results are observed when 
NaCl, for example, is added to gelatine containing HCl, NaOH, AICI3, 
CaCl2, Na2S04 or Nas-citrate. 
The molecular proportions of salts at the minimum, i.e. at the iso- 
electric point or point of greatest precipitability, appears to be char- 
acteristic of the particular salts used. Thus for NaCl and CaCl2, it is 
about 100 to 20; for NaCl and AICI3, about 100 to 1.5. 
Mixtures of NaCl and AICI3 in the above proportions^ gives maximum 
precipitation in gelatine, milk, and olive oil emulsions.^ Gelatine con- 
taining NaCl 0.5M shows maximum precipitation in presence of 
AICI3O.OO78M and Ce(NO)3 0.00625M. The molecular proportions at 
these two points are 100 to 1.5 and 100 to 1.2. Mines^° has observed 
a maximum precipitation of blood corpuscles in NaCl plus CeCl3, 
the molecular proportions being between 100 to 3.3 and 100 to 0.03. 
