The above examples refer to the union in single proportion of each, and are called Binary Compounds. When more than one atom of each element exists in different proportions we have different terms to express such union. If one atom of oxygen be in the compound it is called a “monoxide” or “protoxide”; two atoms of oxygen in combination is termed “dioxide” or “binoxide”; three, “trioxide,” or “tritoxide”; four is the “tetroxide” or “per-oxide,” etc. When more than one atom, but not two atoms is involved, we speak of the sesqui-oxide (one-and-a-half),—“oxide” being interchangeable for “sulphide” or “chloride,” according to the element.
There are other distinctions adopted when metals form two series of combinations, such as ous and ic, which apply, as will be seen, to acids. Sulphuric and sulphurous acids, nitric and nitrous acid are familiar examples. In these cases we shall find that in the acids ending in “ous” oxygen is present in less quantity than in the acids ending in ic. The symbolic form will prove this directly, the number of atoms of oxygen being written below,
Sulphurous Acid = H2 SO3
Nitrous Acid = HNO2.
Sulphuric Acid = H2SO4.
Nitric Acid = HNO3.
Whenever a stronger compound of oxygen is discovered than that denominated by ic, chemists adopt the plan of dubbing it the per (ὑπέρ over), as per-chloric acid, which possesses four atoms of oxygen (HClO4), chloric acid being HClO3. The opposite Greek term, ὑπὸ (hupo, below), is used for an acid with less than two atoms of oxygen, and in books is written “hypo”-chlorous (for instance). Care has been taken to distinguish between the higher and lower; for “hyper” is used in English to denote excess, as hyper-critical; and hypo might to a reader unacquainted with the derivation convey just the opposite meaning to what is intended.
While speaking of these terminations we may show how these distinctive endings are carried out. We shall find, if we pursue the subject, that when wehave a salt of any acid ending in ic the salt terminates in “ate.” Similarly the salts of acids ending in ous, end in “ite.” To continue the same example we have—
Sulphurous Acid, which forms salts called Sulphites.
Sulphuric Acid, which forms salts called Sulphates.
Besides these are sulphides, which are results of the unions or compounds of elementary bodies. Sulphites are more complicated unions of the compounds. Sulphates are the salts formed by the union of sulphuric acid with bases. Sulphides or sulphurets are compounds in which sulphur forms the electro-negative element, and sulphites are salts formed by the union of sulphurous acids with bases, or by their action upon them.
Fig. 322.—Combinations of elements.
The symbolical nomenclature of the chemist is worse than Greek to the uninitiated. We frequently see in so-called popular chemical books a number of hieroglyphics and combinations of letters with figures very difficult to decipher, much less to interpret. These symbols take the place of the names of the chemical compounds. Thus water is made up of oxygen and hydrogen in certain proportions; that is, two of hydrogen to one of oxygen. The symbolic reading is simple, H2O, = the oxide of hydrogen. Potassium again mingles with oxygen. Potassium is K in our list; KO is oxide of potassium (potash). Let us look into this a little closer.