Voi,. 7, 1921 CHEMISTRY: J. KENDALL 61 
univalent salts are practically equally dissociated in aqueous solution, 
yet hydration values so far as we can judge from present data are widely 
divergent. It is a very striking fact, however, that with mercuric salts 
(the one series in which ionization varies considerably) all highly ionized 
salts yield hydrates (e. g., Hg(N0 3 ) 2 , SH 2 0; Hg(ClOi) 2 , QH 2 0; HgF 2 ,2E 2 0) 
while all slightly ionized are non-hydrated. 
It appeared that the problem might be taken up most profitably in two 
stages — by the examination of systems of the type RX-HX (acid salts) 
and of the type RX-ROH (basic salts). By comparison of the results 
in these fields with those already obtained for systems of the types: 
HX-H 2 0 and ROH-H^O it was hoped that generalizations for the more 
complex type RX-H 2 0 might be successfully formulated. The results 
obtained from a detailed investigation of two series of acid salts (formates 
and sulfates) have justified this procedure. 
Acid Salts. — Compound formation in the system acid-salt is found to be 
directly dependent upon differences in electroaffinity, as before. With 
salts of highly electropositive metals (e. g., K, Na) extensive compound 
formation is obtained. As the metal approaches hydrogen in the electro- 
motive series, compound formation decreases rapidly (with Ba, Ca, Mg) 
and finally becomes negligible (with Ni, Fe, Cu). With highly electro- 
negative metals, however (Hg, Ag), compound formation once more be- 
comes appreciable. Ionization again proceeds in parallel with com- 
pound formation, the best conducting solutions in pure formic or sulfuric 
acid being given by the salts of K and Na. 
Solubility. — A new and very significant relationship which appears in 
these systems is that of solubility, which also parallels compound forma- 
tion. The sulfates of K and Na are exceedingly soluble in pure H 2 SOi, 
those of Ba, Ca and Mg are decreasingly soluble, those of Ni, Fe, Cu 
practically insoluble. With electronegative metals the solubility again 
becomes appreciable (with Hg) and finally extensive (with Ag). The 
solubilities of the formates in formic acid follow exactly the same course. 
That the rule is valid also in aqueous solutions may be demonstrated 
by applying it to systems of the type ROH-HOH. The hydroxides of the 
alkali metals are very soluble in water, those of the alkaline earths de- 
creasingly soluble, those of Ni, Fe, Cu practically insoluble. Silver again 
shows an increase, limited however by precipitation of the more stable 
oxide. 
In systems of the type RX-HOH, finally, the same rules hold if R is not 
very different in character from H, or if X is not very different in character 
from OH. Thus the salts of weak bases (e. g., ferric salts) or of weak acids 
(e. g., fluorides) show regular gradations of solubility in water, which 
substantially follow the generalizations given above. For salts of a strong 
acid with a strong base, however, where R and X both differ widely in 
