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 salts present in solution, displacing a definite 

 amount of each metal from combination with chlorine, thus setting free 

 hydrochloric acicl 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 bases 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 Thomson'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 be 

 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 the hydrochloric acid is free and four-fifths of the 

 oxalic acid. 



Maly 3 has also shown qualitatively, by 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 " Thermochemische Untersuchungen," Ann. a. Phys. u. Chem., Leipzig, 1869-71, 

 Bde. cxxxviii.-cxliii. 



' 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 HN0 3 , c that when it partially combines with a mixture of one 

 equivalent of HC1 and one equivalent of HN0 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 

 NaOH unites with HC1, a x will be that set free when the fraction x combines ; similarly 

 b (I -x) will he the amount set free by the combination of the fraction (1 -x} with HN0 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 l-x can be determined. Their ratio is the measure of the 

 avidity of the two acids for combining with the base. 



2 Ann. d. Chem., Leipzig, 1874, Bd. clxxiii. S. 250; Sitzungsb. d. k. Akad. d. 

 Wissenscli., Wien, 1874, Bd. Ixix. Abth. 3, S. 251 ; Ztschr. f. yflnjKiol. Chcm., Strassburg, 

 1877, Bd. i. S. 174. 



