722 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 2 8 



additively compos.ed of two terms, one depending on the positive 

 radical and the other on the negative radical. But in extremely 

 dilute solutions of salts the value for negative radicals was nearly 

 the same; therefore, according to Arrhenius, "the molecular con- 

 ductivity of the active part of an acid (in dilute solution) is con- 

 stant and independent of the nature of the acid," and as a corollary 

 from this "the better the (dilute) solution of an acid conducts 

 electricity, the greater is its active part." For want of precise data 

 for calculating the absolute value of the coefficient of activity, Arrhe- 

 nius takes it as proportional to the molecular conductivity. Thus 

 he is enabled to compare the activities of acids amongst themselves, 

 and of bases amongst themselves. He finds at once that the activi- 

 ties of acids as thus determined from their conductivities agree well 

 with our general notions regarding their strengths, and is led to the 

 statement that " for acids and bases galvanic activity is accom- 

 panied by chemical activity." He proceeds to discuss double de- 

 composition in electrolytic solutions, on somewhat hypothetical 

 grounds, and arrives at a formula (containing coefficients of activity) 

 which he applies practically to many important reactions. If in 

 the general equation of double decomposition 



AB + CD ^ AC + BC, 



1—x n—x y+x g+x 



are the molecular proportions at equilibrium, and a, 8, ^, y are the 

 coefficients of activity of the various substances, then at equilibrium 

 {l — x){n — x) aS= (p + x) {q + x)^y. If the action considered 

 is Acid + Base ^ Salt + Water, the product of the coefficients of 

 activity on the left is, when acid and base are strong, enormously 

 greater than the product of those on the right, and salt-formation is, 

 therefore, practically complete. If acid or base is weak, the two 

 products are comparable, and in consequence, entire neutralization 

 will not take place, notable quantities of acid and base remaining 

 free. If the activity coefficient of one of the substances (say alcohol 

 regarded as acid) is smaller than that of water, only traces of the 

 salt are formed. Here we find a definite treatment of salt-hydroly- 

 sis based on the following principle : " What is common to all these 

 cases is the necessity of regarding water as an acid (or as a base) 

 which competes with other acids (or bases) present in the equilib- 

 rium." Arrhenius states further the proposition, which requires 

 some restriction, that " at a dilution not excessively great the quan- 

 tity of salt decomposed is appi'oximately proportional to the square 

 root of the quantity of the solvent water." 



The theory is then applied to the displacement of one acid by 

 another, to the influence of acid salts, and to equilibrium in hetero- 

 generous systems. The consequences of the variation of the coefficient 



