Chemical Nomenclature. — In early times chemical substances were named according to the fanciful theories of Al chemy. Thus the name "flowers of sul: phur" was applied to sulphur (sublimed), which grew or sprang like a flower from sulphur when heated; "spirit of salt," to hydrochloric acid, the corrosive acid or spirit obtained from common salt; and a multitude of other names had a fanciful origin. In 1787 Lavoisier founded the system of nomenclature still followed by chemists. At first it was intended that the name of a simple as well as of a compound substance should be regulated by system, and that it should indicate the nature of its elementary constituents, as well as the relative proportion in which they were present. Hence such terms as oxygen, the acid-producer, given from the notion then held that no acid was without oxygen; and hydrogen, the water-producer, from the supposition that hydrogen had more to do with the formation of water than any other ele ment. The advance of chemistry, how ever, so completely changed the opinion of chemists regarding the simpler bodies that such names were found to mislead; and thereafter, though such as had been given on this system were retained, their meaning was discarded, and the system atized nomenclature restricted to coin, pound substances. In the non-metallic elements a close analogy 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 on. 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 um. The symbol of an element is obtained from the abbreviation or first letter of its Latin name, as 0 for "oxygen"; Pb for "lead" (Lat. plumbum). 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 nitrate, 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 trichloride and phosphorus penta chloride, respectively. Similarly, carbon monoxide contains one atom of oxygen, carbon dioxide, two atoms, and so on.
Chemical Syrnbols.—A symbol denotes one atom of the element. Thus, 0 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 02. But the ozone molecule contains three atoms of oxygen and is written 03.
The combination of two elements is represented by placing the symbols for those elements side by side; thus, H2O signifies two atoms of hydrogen and one atom of oxygen in a state of chemical combination (viz., water), and NaCI 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 MnO, is one atom of manganese with two of oxygen (black oxide of manganese), Fe:02 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 FeSO,. 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 NH,, 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 saw above, that the formula for ferrous sulphate was FeSO, The formula for ammonium sulphate is (NH4)2SO4 and, to express the fact that the atoms of nitro gen and hydrogen forming ammonium act as a single entity, parentheses are used, as shown. These symbols and for mule are valuable and necessary because they enable us to express briefly the exact composition of a substance, and because, by their use, chemical reactions can be shortly expressed in the form of an equation. Thus: CaCO3 H2S0, = CaSO, + H2O + CO2 indicates that calcium carbonate reacts with sulphuric acid to give calcium sulphate, water and carbon dioxide.
Metals.—Largely from their physical characteristics, a number of the elements are called metals. They all possess the metallic luster, are of opposite affinity to oxygen, can within certain limits for each case replace hydrogen in acids and other metals in salts. They conduct electricity and heat comparatively well, and are gen erally solid at an ordinary temperature.
Non-metals.—All the other elements are classed as non-metals. Some are solid, such as sulphur and iodine; bro mine is liquid, and many are gaseous at ordinary temperatures, such as oxygen and chlorine. Some elements are on the border line, such as silicon and arsenic, it being hard to class such definitely as metals or non-metals.
Chemical Laws.—By many years of patient research the chemists of the late 18th and early 19th century estab lished the fact that all chemical re actions take place in accordance with definite quantitative laws. The first and simplest chemical law is known as the Law of Constant Proportions. This states that: "The same compound always con tains the same elements combined to gether in the same proportion by weight." This law establishes the difference be tween a chemical compound and a mix ture. A mixture may obviously be com pounded of its ingredients in varied proportions. Putty, for instance, may contain more or less whiting, more or less oil, and still it will be putty. But cal cium carbonate, which is the chief con stituent of whiting, will always contain calcium, carbon, and oxygen combined in the same proportions. It may be dug from the earth as calcite, or prepared artificially by treating lime with car bonic acid, or obtained as a precipitate by treating sodium carbonate solution with calcium chloride, but, however ob tained, it will always contain 40 parts of calcium to 12 parts of carbon and 48 parts of oxygen.