according to the standpoint from which they are viewed. These bodies form a connecting link between metals and non-metals, making it impossible to draw a strict line of demarcation. The influence of heat upon metals is very varied; some melt at a low temperature, others require a red heat, a strong red, or a white heat respectively, to melt them. The following table by Pouillet will explain the temperatures corresponding to different colours : Metals expand when heated and contract on cooling, and, within certain limits, the expansion is proportional to the degree of heat. Certain anomalies, however, exist, thus: molten cast-iron expands at the moment of becoming solid, and solidified bismuth occupies a larger space than bismuth in the liquid state. One of the most distinctive features of a metal is an internal mobility, in virtue of which its shape may be altered by pressure without disruption of the mass. This property is possessed by metals in various degrees, so that the "malleability" or capability of being extended by pressure without cracking, and "ductility," or the capability of being permanently elongated by a tensile stress combined with lateral pressure, are by no means equal in extent; nor is the order of their malleability the same as for ductility, for the former depends on the softness and tenacity, while the latter is much more dependent on tenacity. By "tenacity" or tensile strength is understood the resistance a body offers to an attempt to pull its particles asunder when a stretching force is applied. The tenacity is generally diminished by a rise in temperature, while the reverse is often the case with regard to malleability and ductility. Some metals have a feeble tenacity, and are then said to be brittle. When a body resists rupture by a bending or twisting force, it is said to be "tough." Elasticity is a property of bodies, in virtue of which they tend to recover their size and shape after being subjected to a disturbing force. There is a limit in every solid body, beyond which it will not return to its original form on the withdrawal of the force; this is termed the limit of perfect elasticity. "Hardness," which is measured by resistance to a compressive force, like all the other physical properties of a metal, is modified by the presence of impurities, so that, in many cases, softness is a test of purity. All malleable metals become hardened by pressure, and often require annealing during the process of manufacture. The fractured surface of metals is often characteristic, being spoken of as fibrous, crystalline, granular, columnar, and conchoidal, thus: wrought-iron is fibrous, zinc is crystalline, steel is granular, tin is columnar, and hard steel is conchoidal. Crystalline structure is often accompanied by brittleness, and fibrous structure by high tenacity. Most metals are much heavier than water, and the ratio which expresses the number of times a body is heavier than an equal volume of water is termed its "specific gravity." The "specific heat" or the capacity of a body for heat extends over a wide range with regard to metals; iron, for example, being that of water, while lead is less than 1 The oxides of metals are usually basic in character, but this property is only relative, as an oxide which is basic in one compound may become acid when allied with a stronger base. CLASSIFICATION OF METALS § 3. Metals may be more conveniently studied by classify ing them into groups, according as they approximate in general characters, based chiefly on their affinities for oxygen. 1. Noble Metals.-Gold, Platinum, Silver, and a few rare metals. The members of this class have little or no tendency to unite with oxygen in the free state, and when placed in water at a red heat do not alter its composition. The oxides are readily decomposed by heat in consequence of the feeble affinity between the metal and oxygen. 2. Copper Group.-Copper, Mercury, Lead, and Bismuth. They do not decompose water, even at a red heat, but unite with oxygen when heated, forming oxides of a basic character. Mercury has the least affinity for oxygen, and in this respect approaches the noble metals in character. 3. Tin Group.-Tin, Antimony, Arsenic, etc. These metals, except arsenic, decompose steam at a red heat, and form some oxides of an acid character; in this respect resembling the non-metals. Protoxide of tin is a strong base. 4. Iron Group.-Iron, Chromium, Manganese, Nickel, and Cobalt. Steam is decomposed when brought in contact with the members of this group at a red heat. The protoxides form powerful bases. Some of the higher oxides of iron, chromium, and manganese form what may be termed metallic acids, which combine with bases to form salts, soluble in water. 5. Zinc Group.-Zinc, Cadmium, and Magnesium. The members of this group oxidise slowly in moist air, but rapidly when heated. They are readily volatile, and burn in air with a bright light, producing oxides, which form powerful bases. At a red heat they decompose steam. 6. Aluminium Group.- Aluminium and allied rare metals. Aluminium does not oxidise in air or decompose water, unless the metal is in a very finely divided state. It forms an oxide, which is acid or basic according as it is allied with a stronger or weaker base. 7. Alkaline-earthy Metals.-Barium, Strontium, and 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." com 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 :— |