214 OUTLINES OF PHYSICAL CHEMISTRY
clearly that in aqueous solution ions have an independent existence.
Silver nitrate gives, with a solution of any metallic chloride, a white precipitate of silver chloride. But if silver nitrate he added to a solution of potassium chlorate or of monoohloracetic acid the silver does not unite with the chlorine. Ferrous salts give a black precipitate with ammonium sulphide, but this same reagent gives no black precipitate with potassium ferrocyanide. From these examples, which might be greatly multiplied, we can see that the reactions take place between ions. A silver ' solution is not a reagent for chlorine, but for the chlorine ion ; ammonium sulphide is a reagent for the ferrous ion. Analytical chemistry is based principally on reactions of double exchange between ions. There are no reagents for such and such an element, but for such and such an ion.
It must be noticed that ions behave in a different manner according to their valency. The iron ion, for instance, behaves in two different ways according as it is derived from a ferrous or from a ferric salt. In the same way the radical FeCy 6 behaves in different ways according as it is present as a ferrocyanide (E 4 FeCy 6 ) or as a ferricyanide (K 3 FeCy 6 ).
These are the most important facts which can be produced in favour of Arrh&nius's doctrines, and I shall now devote a few lines to a critical examination of this hypothesis.
Above everything else we notice that the hypothesis of free ions is in opposition to thermo-chemical observations.
Let us recall that hydrogen and chlorine in uniting evolve much heat, and that the solution of hydrochloric acid gas in water is strongly exothermic.
In spite of this, if the theory of ArrMnius be true, no closer union of the atoms has taken place, for in the solution the dissolved acid is in the state of free hydrogen and chlorine ions. Again, the heat of formation of caustic soda is very large and its heat of solution is by no means negligible ; and further, a solution of caustic soda neutra-
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