_The_ Acid _Oxides._--This class, taking the stronger acids as the type, may be described as follows:--very soluble in water, the solution possessing an intensely sour taste, and a _corroding_ rather than a solvent action upon the skin; changes the blue colour of litmus and other vegetable substances to red, and neutralizes the alkalies and basic oxides generally. Observe however that these properties are possessed in very various degrees by different acids. Prussic Acid and Carbonic Acid, for instance, are not sour to the taste, and being feeble in their reactions, redden litmus scarcely or not at all. All acids however, without any exception, tend to combine with bases and to neutralize themselves; so that this may be said to be the most characteristic property of the class. _Chemical composition of Acid and Basic Oxides contrasted._--It is a law commonly observed, although with many exceptions, that bases are formed by the union of Oxygen with _metals_; and acids, by Oxygen uniting with _non-metallic elements_. Thus, Sulphuric Acid is a compound of Sulphur and Oxygen; Nitric Acid, of Nitrogen and Oxygen. But the alkali, Potash, is an oxide of the _metal_ Potassium; and the oxides of Iron, Silver, Zinc, etc. are bases, and not acids. Again, the composition of acids and bases is different in another respect; the former invariably contain more Oxygen in proportion to the other element than the latter. Taking the same examples as before, the two classes may be represented thus:-- Acids { Oil of Vitriol, Sulphur 1 atom, Oxygen 3 atoms. { Aqua-fortis, Nitrogen 1 " Oxygen 5 " Bases { Oxide of Silver, Silver 1 atom. Oxygen 1 atom. { Oxide of Iron, Iron 1 " Oxygen 1 " The class of Hydrogen Acids.--Oxygen is so essentially the element which forms the acidifying principle of acids, that its very name is derived from that fact (οξυς, acid, and γενναω, to generate). Still there are exceptions to this rule, and in some acids _Hydrogen_ appears to play the same part; the _Hydracids_, as they are termed, are formed principally by Hydrogen uniting with elements like Chlorine, Bromine, Iodine, Fluorine, etc. Thus, Muriatic or Hydrochloric Acid contains Chlorine and Hydrogen; Hydriodic Acid contains Iodine and Hydrogen. Observe, however, that the position held by the Hydrogen in these compounds, is different from that of the Oxygen in the "Oxyacids," as regards the number of atoms usually present; thus-- Aqua-fortis = Nitrogen 1 atom, Oxygen 5 atoms, Muriatic Acid = Chlorine 1 " Hydrogen 1 atom; so that the composition of the Hydracids is analogous to the _basic_ oxides, in containing a single atom of each constituent. THE TERNARY COMPOUNDS OF THE ELEMENTS. As the various elementary substances unite with each other to form Binary Compounds, so these binary compounds again unite and form _Ternary_ Compounds.