A SOLID is a substance the molecules of which are more or less immobile, though probably not in absolute contact. A LIQUID is a substance the molecules of which so freely move about each other that it readily assumes and retains the form of any vessel in which it is placed. A GAS is a substance the molecules of which are so far apart that they seem to have lost all attraction for each other, and, indeed, to have acquired the property of repulsion to such an extent that they are only prevented from receding to a still greater extent by the pressure of surrounding matter. Motion is especially characteristic of gaseous fluids. THE VOLUME OF A GAS depends altogether upon its temperature and pressure. The lower the temperature and the higher the pressure the less its volume. There are three fundamental principles which apply to all true gases. They are known as: THE LAW OF AVOGADRO.-Equal volumes of all gases, at the same temperature and pressure, contain the same number of molecules. THE LAW OF BOYLE OR MARRIOTTE.-The volume of a given weight of gas, the temperature remaining the same, will be inversely as the pressure upon it. THE LAW OF CHARLES.-The volume of a given weight of gas, the pressure being unchanged, will vary directly as the absolute temperature. MOLECULAR WEIGHTS.-These are the weights of equal volumes of gases or vapors, under equal circumstances of temperature and pressure, and relative not to I, but to 2 of hydrogen. In the case of non-volatile bodies, molecular weight is deduced from the observed analogies of the bodies with those whose molecular weight admits of proof. The molecular weight of a compound is the sum of the atomic weights. ATOMIC WEIGHTS are, first, the proportions in which elements are found to combine with each other by weight. (The figures showing these proportions are purely relative, but all chemists agree to make this relation fixed by giving the number I to hydrogen.) Secondly, they are the weights of equal volumes of elements in the state of gas (relative to 1 of hydrogen). The principal varieties of chemical compounds are Oxides, Hydrates and Salts. A HYDRATE is the combination of a Base with Hydroxyl (OH), which is a hypothetical union of oxygen and hydrogen. AN OXIDE is the combination of a Base with oxygen. A SALT is the combination, either wholly or in part, of a base with an acid radical. There are three kinds of salts. NORMAL, or Neutral Salts, where all the replaceable hydrogen of the acid is replaced by the base. ACID SALTS, where but part of the hydrogen is replaced by the base. BASIC SALTS, where the hydrogen is replaced in part by the base, and in part by an oxide or hydrate of the base. The term Salt is also applied to most solid chemical substances, but more especially to those having a crystalline form. Some metals form two or more classes of salts, according to their valencies, in which case the name of the salt in which the element has the lower valence ends in "ous," the higher in "ic," as "arsenous" and "arsenic." A BASE is a substance which replaces the hydrogen of an acid and forms a salt. The direct union of two elements or salts, whose name end in "ide," such as Iodides, Bromides, Chlorides and Sulphides. AN ACID is a chemical compound formed by the union of hydrogen with an acidulous radical, and forms salts by having its hydrogen replaced by a base. All acids must contain hydrogen. Farther, acids may or may not contain oxygen. Those which contain no oxygen are called Hydracids, those which do contain oxygen are termed Oxyacids. Acids containing carbon are styled Organic acids. (Carbonic acid, however, is generally classed with the Inorganic acids.) The names of salts from the "ous" oxyacids end in "ite," as nitrite (from nitrous acid), sulphite (from sulphurous acid); those from "ic" oxyacids. end in "ate," sulphate (from sulphuric acid), nitrate (from nitric acid), etc. (As a general thing the names of salts from the organic acids end in "ate.") Anhydrous bodies (i. e. without water) are compounds from which water has been taken, but whose essential chemical properties are unaltered. Salts containing water are hydrous bodies; of these the larger portion are crystalline, and their water is then termed water of crystallization. Non-crystalline hydrous compounds were formerly spoken of as hydrated substances; hydrates are, however, a distinct class of bodies; salts derived from water by one-half of its hydrogen becoming displaced by an equivalent quantity of another radical. Anhydrides form still another distinct class of chemical substances; they are derived from acids; in short, they are acids from which, not exactly water as water, but the elements of water have been removed, the essential chemical (acid) properties being thereby greatly altered. The following prefixes are used in the nomenclature of salts and acids: super- and hyper, meaning above, over, in excess. per, meaning thorough, to full extent. sesqui-, meaning one and a half. hypo-, or sub-, meaning below or under. proto-, meaning first or lower. ortho-, meaning original or regular. meta-, meaning beyond, derived from. pyro-, meaning produced by high heat. Bi, Bin and Dueto, used for double; as Bi-carbonate, Bin-oxalate, Dueto-iodide. * |