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LEFT CHEMISTRY 447 CHEMISTRY pound substances. In the non-metallic elements a close analog^y exists between chlorine, bromine, iodine, fluorine; and to indicate this the common termination ine has been given; and for a similar reason carbon, silicon, and boron end in an. As a general rule, however, the chemical name of an elementary sub- stance does not convey any scientific meaning, and must be regarded as a simple mark or designation, analogous to the names of persons, which give no no- tion regarding their moral character or physical development. The ancient and more common metals retain their popular titles, such as gold, silver, and copper; but the more recently discovered metals have names given which end in uvt. The symbol of an element is obtained from the abbreviation or first letter of its Latin name, as O for "oxygen"; Pb for "lead" {Lat. plumbiun) . When the names of two or more elements commence with the same letter a smaller letter or satel- lite is attached to one or more of these; such as S for "sulphur," Se for "selenium," and Si for "silicon." The most common of these are O., oxygen; H., hydrogen, and N. nitrogen. The name of a compound substance generally indicates the elements of which it is composed. Thus the name "ferric oxide" indicates that the red powder is made up of oxy- gen and iron; the name "lead sulphide" (galena) that it is composed of sulphur and lead. In the first case the adjective is derived from the Latin name of the metal. When two elements combine with each other in more than one proportion or equivalent, the names of the com- pound bodies are contrived to express this. Thus, oxygen combines with a number of elements to produce with each a series of acid compounds, the more highly oxidized of which receive the ter- mination ic, while those containing less oxygen end in ous. Thus sulphuric acid contains three equivalents of oxygen to one equivalent of sulphur, and sulphur- ous acid two equivalents of oxygen with one equivalent of sulphur. "Ferrous chloride" indicates the lower chloride, and "ferric chloride" the higher chloride, of the metal iron. When acids combine with bases or metallic oxides to form salts they produce compounds the names of which are influenced by the terminations of the acids. Thus, sulphuric acid and sodium form sodium sulphate; sulphur- ous acid and sodium, sodium sulphite. In the same manner nitric acid combined with potassium forms potassium niti'ate, while nitrous acid and potassium produce potassium nitrite. Another method, and one which is growing in favor, is the use of prefixes. Thus, phosphorus unites either with three or five atoms of chlorine and the compounds are known as phos- phorus t7t'chloride and phosphorus penta^ chloride, respectively. Similarly, carbon ?no?ioxide contains one atom of oxygen, carbon dioxide, two atoms, and so on. Chemical Sytnbols. — A symbol denotes one atom of the element. Thus, sig- nifies one atom, or 16 parts by weight, of oxygen; C, one atom, or 12 parts by weight, of carbon; H, one atom, or one part by weight, of hydrogen. A molecule is composed of various numbers of atoms. For instance, a mole- cule of hydrogen contains two atoms, and its symbol is therefore written H2. The molecule of oxygen also consists of two atoms and is written O2. But the ozone molecule contains three atoms of oxygen and is written O3. The combination of two elements is represented by placing the symbols for those elements side by side; thus, H3O signifies two atoms of hydrogen and one atom of oxygen in a state of chemical combination (viz., water), and NaCl is one atom of sodium (Lat. natron) united with one atom of chlorine (viz., common salt). When two or more atoms of one element unite with one or more atoms of another element the number of such atoms is signified by a small figure placed immediately after the symbol of the ele- ment so multiplied. Thus MnOn is one atom of manganese with two of oxygen (black oxide of manganese), FesOa is two atoms of iron with three atoms of oxygen, and Pb304 is three atoms of lead with four atoms of oxygen (red lead). In ex- pressing the formula of a compound sub- stance the symbol of the metal or its analogue is generally placed first in order, and is succeeded by the oxygen, chlorine, or similar element. The same order is carried out in the construction of the formula of more complex sub- stances; the metallic half is placed first. Thus, ferrous sulphate — containing sulphuric oxide and the oxide of iron — is generally expressed as FeSOi. In other words, the symbols are written in the order in which the substances would be named in Latin. Some substances contain so-called com- pound radicles, consisting of two or more atoms, combined together to act as a single atom within the molecule. Am- monia is a well-known example of this. The formula for ammonia gas is NH3, and in aqueous solution it combines with water to form ammonium hydroxide, (NH,) OH. The radicle (NH.) exists in many compounds and whereas it is plainly built up of one atom of nitrogen and four of hydrogen, it acts as though it were a single atom. For instance, we