Calcium. These metals rapidly decompose water, forming oxides. All the oxides are strongly basic, and readily unite with acids.

8. Alkali Metals.-Sodium, Potassium, and Lithium. These metals have such an intense affinity for oxygen that they require to be preserved in some liquid free from that element. The oxides dissolve in water forming powerfully caustic solutions.

NATURE OF NON-METALS

§ 4. All the chemical elements devoid of that combination of physical and chemical properties which constitute the metals are termed non-metals. Only a few of them are found in nature in the free state; namely, oxygen, nitrogen, sulphur, and sometimes carbon and selenium. As a general rule they are opposite to the metals in their electrical relations, being electro-negative, and, consequently, appearing at the positive electrode when their compounds are decomposed by the electric current. The positive or negative character, however, like that of the metals, is not absolute, except in the elements oxygen and fluorine, but varies according to the kind of elements existing in combination. Thus, in the compound known as hydrochloric acid, chlorine is negative; but when chlorine is in combination with oxygen it plays the part of a positive element.

The combination of a metal with a non-metal, such as oxygen, is characterised by a much more decisive change of properties than in the case of union between two metals. The affinity of the different metals for oxygen, sulphur, etc., is however very varied, being more intense the wider the difference between the electrical relations of the constituents. In the case of gold the affinities are so feeble that this metal does not unite directly with either of these non-metals. The compounds of most metals with oxygen form a class of bodies termed bases; while the combination of non-metals with oxygen generally forms a class of bodies of opposite

properties, termed acids. Those metals, however, which exhibit the metallic character in a feeble degree, more often have acid than basic properties when combined with oxygen. It is upon these chemical relations, taken in conjunction with the physical properties, that a true classification of the elements into metals and non-metals is based.

CHEMICAL PRINCIPLES AND CHANGES

§ 5. Chemists have found that all bodies, whether in the form of a solid, a liquid, or a gas, are either simple substances or can be resolved into simple substances, termed "elements." These elements are represented by symbols, which are usually the initial letter or letters of their names. Different elements combine together in definite proportions, forming an endless variety of substances termed " pounds."

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Elements are classified into "metals" and "non-metals," the former being distinguished by well-marked properties, which are absent in the latter. The ultimate particles or "atoms" which compose any element differ in weight from the atoms of any other element, and the relative weight compared with hydrogen is termed the "atomic weight." The following table gives the chemical elements with their symbols, atomic weights, and specific gravities:

Compounds are formed, as already stated, by the combination of different elements, thus: FeO represents oxide of iron, and MnO oxide of manganese. In many cases two elements unite in more than one proportion, such as FeO, Fe2O3, Fe3O4, each of which requires a distinguishing name. There are several systems of nomenclature, but the simplest-for compounds containing two elements-is that of writing the name of the metal first, and the non-metal or least metallic element afterwards, giving it the termination "ide." When two non-metals combine, the one which is most unlike a metal is written second. Sometimes Greek prefixes are used for the element of the second position, such as mono, di, tri, tetr, etc., to indicate the number of atoms present. Another system is to make the metal terminate in "ic" or 'ous." That compound which contains the greater proportion of the non-metallic constituent is distinguished by the suffix "ic" and that containing the smaller by the termination "ous." The following list will illustrate these points :

When three elements-one being a metal and another oxygen-are combined together, the name of the second is made to end in "ate.' In the following list a few compounds are given to illustrate this, but it should be observed that the order of placing the symbols is immaterial. In works on metallurgy the arrangement of formulæ in the last column is most common.