THE ACID OF THE GASTRIC JUICE. 357 
as the phosphoric acid passes into solution, it, no Longer remains present 
as free phosphoric acid, to the amount to which it has been added, but 
reacts with the other Baits present in solution, displacing a definite 
amount of each metal from combination with chlorine, thus setting 
hydrochloric acid and forming phosphates, so that there comes to be in 
solution free hydrochloric acid and free phosphoric acid, combined 
phosphoric acid, and combined hydrochloric acid (that is, chlorides and 
phosphates). "When a polybasic acid, such as phosphoric acid, is present 
in solution, the matter is somewhat further complicated by there being 
certain steps between free acid and combined acid, namely, acid salts ; 
these also are represented in the distribution of liases among the acids, 
so that there are in solution free acids, acid salts, and neutral salts. In 
pure gastric juice, then, the acidity is in chief due to hydrochloric acid, 
but also in part to acid phosphates and phosphoric acid, and the amount 
of each of these free is perfectly determinate, and depends upon the 
amount of each base and each acid present. For one fixed distribution 
only can there be chemical equilibrium in the solution : the introduction 
of any salt, acid, or base into the solution will alter this equilibrium, and 
a new distribution to suit the new conditions will occur, giving rise 
again to equilibrium. 
The facts stated above follow directly from Thomsen's 1 " avidity 
law." Thomsen arrived at this law by comparing the amount of heat 
set free when an equivalent weight of a base unites with a mixture of 
equivalent weights of two different acids, with the amount set free when 
it combines with each acid separately. 2 The law is that no acid in solu- 
tion is combined with the bases present, to the complete exclusion of 
other acids, however weak (as it is popularly expressed), which may lie 
simultaneously present in the solution : but the acids share the bases, 
according to their different avidities. Thomsen worked out a number of 
avidity coefficients. Those of the organic acids are much smaller than 
those of the inorganic acids. Thus, taking the avidity coefficient of 
hydrochloric acid as unity, that of oxalic acid is - 25, tartaric acid '05, 
acetic acid "03. These coefficients mean, for example, that if one 
equivalent each of sodic hydrate, of hydrochloric acid, and of oxalic acid, 
be mixed in solution together, four-fifths of the base is combined with 
the hydrochloric acid and one-fifth with the oxalic acid, and con- 
sequently one-fifth of tire hydrochloric acid is free and four-fifths of the 
oxalic acid. 
Maly 3 lias also shown qualitatively,^ a method of diffusion, that 
this displacement of a strong acid (i.e. acid with a large avidity co- 
efficient) by a weak acid (acid with a small acidity coefficient) takes 
1 " Thermoehemische Untersuchungen," Ann. a. Phys. it. Chem., Leipzig, 1869-71, 
Bde. cxxxviii.-exliii. 
2 Let a be the amount of heat in heat units developed when, say, one equivalent of 
NaOH in grammes combines with one equivalent of HC1, and b that when it combines 
with an equivalent of HX0 3 , c that when it partially combines with a mixture of one 
equivalent of HC1 and one equivalent of HX0 3 , also let x be the fraction which combines 
with HC1. Then, since a is the amount of heat set free when a whole equivalent of 
XaOH unites with HC1, a x will be that set free when the fraction x combines ; similarly 
b (1 —x) will be the amount set free by the combination of the fraction (1 -x) with HX0 3 ; 
the sum of these two must equal c, the amount of heat actually observed ; therefore a x + 
b (1 -x) = c, from which x and 1 - x can be detennined. Their ratio is the measure of the 
avidity of the two acids for combining with the base. 
3 Ann. d. Cl/em., Leipzig, 1874, Bd. clxxiii. S. 250; Sitzungsb. d. l\ Akad. d. 
isch., Wien, 1871, Bd. lxix. Abth. 3, S. 251 ; Ztschr. f. physiol. Chem., Strassburg, 
1877, Bd. i. S. 174. 
