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Symbols and Atomic Weights of the Elements used in this Volume.







(i) Chemical Distinction of Bodies into Elements and Compounds.—Modern science has shown that, numberless as are the substances presented to us in the daily experience of life, there are very few which cannot be separated into other substances of a less complicated nature. For example, a piece of wood, a sheet of paper, and a quill pen can each be separated into three or four distinct substances; namely—oxygen, hydrogen, carbon, and nitrogen ; but from these we cannot extract any simpler forms of matter. It is the province of chemistry, i. To resolve matter into its simplest components; 2. To ascertain the properties of these simple or elementary forms of matter; 3. To combine two or more of these elementary bodies with each other, so as to form compounds ; 4. To study the properties of these compounds j and 5. To define the conditions under which such compounds can exist.

Bodies which have hitherto resisted all attempts to resolve them into other forms of matter are in chemical language termed elements, or simple substances. In popular language, the word element is often referred to fire, air, earth, and water. A very slight acquaintance with chemistry is sufficient to prove that air, earth, and water are compound bodies, and that fire is mainly the result of the action of a high temperature on certain bodies.

All natural objects then, consist either of simple bodies, or they are composed of two or more simple bodies united according to certain rules or laws which form the groundwork of the science of chemistry. Substances thus produced by the union of two or


more elements are termed compound bodies. These compounds have in general no more resemblance in properties to the elements which have united to form them, than a word has to the letters of which it is made up.

Simple or elementary substances are limited in number. In the present state of the science only sixty-four are known. Many of these are widely distributed, and form in mixture or in combination with others, considerable portions of the earth's crust; others have hitherto been found only in minute quantities. Of these simple substances the larger proportion are metals, many of which are familiar to us, such as gold, silver, copper, iron, lead, tin, and mercury. Other simple substances equally familiar have no resemblance to the metals; such as charcoal, sulphur, and phosphorus. Some simple substances exist in the form of air or gas: such, for example, are the two essential components of the atmosphere, oxygen and nitrogen.

Simple substances have been divided into two great classes— non-metallic* and metallic. The substances comprised in the latter class are the more numerous, but those in the former are the more abundant and the most widely distributed.

The elements enumerated as non-metallic are thirteen in number, —viz.,


Of these substances, oxygen, hydrogen, nitrogen, chlorine, and probably fluorine, are gaseous; bromine is liquid, and carbon, iodine, sulphur, selenium, tellurium, phosphorus, boron, and silicon, are solid at ordinary temperatures.

(2) Difference between Physical and Chemical Properties.— The properties which characterize objects in general, may be classed under two heads—viz., physical and chemical.

• Some writers speak of the non-metallic bodies as the metalloids, a term which signifies metal-like substances. This unfortunate misnomer could never have become even partially current, but for the want of a good single word for the phrase ' non-metallic body.'

f Tellurium is sometimes enumerated among the non-metals, but it more nearly approaches the metals in many of its characters. It possesses a tin white colour and brilliant metallic lustre; its specific gravity is more than 6, whilst that of the heaviest of the non-metals, iodine, is not quite 5. On the other hand, its power of conducting electricity is very low, and it has very great chemical analogies with sulphur and selenium.


The physical properties of an object are those which refer to its condition, whether solid, liquid, or gaseous. Crystalline form, density, hardness, colour, transparency, or opacity, and the relations of the object to heat and electricity are also physical properties. The physical properties of a body have no reference to its elementary composition. The chemical properties of the body, on the contrary, relate essentially to its action upon other bodies, and to the permanent changes in composition which it either experiences in itself, or which it effects upon them. For example, in indicating the physical properties of sulphur, we refer to its brittleness, its crystalline structure, its fusibility and volatility, its faint peculiar odour, its yellow colour, its semi-transparency, its electrical excitability, or the facility with which if rubbed it exhibits electrical attraction, and so on. But in describing the chemical properties of sulphur we indicate its inflammability, its tendency to unite with silver or copper when heated with it, its insolubdity in water and alcohol, its solubility in oil of turpentine and in alkaline liquids. In short we notice those operations by which the body usually becomes changed, and loses its distinctive physical characters.

It is not however always possible to draw the line between physics and chemistry; this is of the less importance, since the chemical nature of any substance could be but imperfectly studied, without a tolerably complete knowledge of its leading physical characters, by which it is most readily defined.

(3) Physical States of Matter.—Natural objects are presented to us in three states, or physical conditions—viz., the solid, the liquid, and the gaseous, aeriform, or vaporous. Every substance exists in one or other of these conditions. The same body may, however, often be made to assume any one of these conditions at different times, and pass from one to the other for an indefinite number of times, according as it is exposed to a greater or less degree of heat. Ice, water, and steam are the same material in three different states. In whatever form matter exists,— whether visible, as in the state of ice or water, or invisible, as in that of steam,—the quantity of matter always remains the same; there can be no destruction of matter. A quantity of ice or of water that weighs a pound, will still, as steam, be equally a pound. So it is with all gases; the air, although invisible, is not the less capable of being weighed and measured.

(4) Porosity.—Natural objects, of whatever form, are composed of particles which are not in actual contact, but are separated by spaces or intervals termed pores. A lump of sugar or of

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