in union with monads produces molecular chains. Oxygen acts extensively in this way, as in and, by introducing more cxygen-links, such chains may be indefinitely extended. With atoms of higher quantivalence the complexity is increased in a still greater degree, the multivalent atom playing the part of a nucleus. The following scheme represents the constitution of common alum as a saturated molecule : The double atom of aluminium is the nucleus of the group, and combines four subordinate groups, each having a nucleus of hexadic sulphur. It matters nothing how such a scheme is drawn, so that the atomicities are all satisfied, but from the way such complex molecules break up in decomposition it is inferred that there must be some definite order of arrangement among the atoms. § 4. Theory of Radicals. 252. Simple Radicals.-The term radical has long been applied to any chemical body which is regarded as a common ingredient, or basis of a series of compounds. Thus potassium, sulphur, and, in fact, any element may be taken as the starting-point, or root, of such a series. The simple radicals, or elements, may be divided into two great classes, which stand in opposite relations, the metals and the non-metals, the former being electro-positive, or positive radicals, and the latter electro-negative, or negative radicals. 253. Compound Radicals.-But it has been established that there are groups of elements so bound together that they play the part of simple bodies, and are therefore called compound radicals; thus carbon and nitrogen combine to form the radical cyanogen CN, which is the root of a series of compounds much resembling those formed by chlorine. Ammonium, NH,, is a compound radical which behaves in chemical reactions closely like the metals, combining with chlorine, sulphur, and cyanogen. Methyl, CH,, is the radical of methylic alcohol; and ethyl, C,H,, is the root of ethylic alcohol, both of which are traceable through numerous affiliated compounds. These compound radicals are classed as positive and negative, like the simple ones. 254. Quantivalence of Compound Radicals.-Compound radicals also obey the laws of quantivalence like simple radicals. In general they cannot be isolated, as they are unbalanced molecules; but some of them pair with each other like elementary atoms, forming saturated molecules which can exist separately. The radical hydroxyl, H−0-, cannot, as it has an unsaturated bond, exist free, but coupled as H-0-0-H it forms the compound known as hydric peroxide. The compound radicals interchange with each other, and with the simple radicals, under the usual limitations of atomicity, or, according to the number of free bonds. As represented by the graphic symbols, the following radicals are monatomic: § 5. Theory of Acids, Bases, and Salts. 255. The Old View.-These numerous and important bodies were long explained in a very simple way on the dual theory, already noticed. The primary elements were divided into the metals and non-metals, which, uniting with each other in pairs, give rise to binary compounds, acids and bases. Acids are sour, corrosive substances, that turn vegetable blue colors to red, and have a strong chemical attraction for bases. Bases, on the other hand, are a class of bodies, (including alkalies, which have a hot, acrid taste, and restore the blues discharged by acids,) that are marked by their powerful chemical attraction for acids. The union of acids and bases gives rise to the ternary compounds known as salts-bodies, generally, with a saline taste, and in which the acid and basic constituents are partially or totally neutralized. For example, the element oxygen, "the centre of the chemical world," and long regarded as the acidifying principle of Nature, unites with sulphur to form sulphuric acid, SO,. Oxygen also combined with potassium to form basic potash, KO. These binaries then paired in the production of the ternary salt, sulphate of potash, KO, SO,. Even in salts affinity is often not exhausted. They may be again coupled, producing quaternary compounds, or double-salts. Most of the bodies of Nature were viewed as composed of these four great groups, primaries, binaries, ternaries, and quaternaries; and chemistry, for half a century, consisted in extending chemical knowledge under the guidance of this system. But, as facts have accumulated, it has undergone a profound modification. 256. Water in Relation to the Theory.-Water was long supposed to act only as a solvent medium, facilitating the reactions of other bodies, but not participating in the changes, except that its particles were sometimes taken up, and appended to other compounds, as "water of hy dration," or water of crystallization." But at length it began to be recognized that the elements of water are themselves seriously implicated in the transformations. It turned out, in fact, that, in regard to the constitution of acids, alkalies, and salts, water holds a controlling relation; its molecule being the pattern upon which they are all constructed. It was, moreover, found that the union of acids and bases in the production of salts is not a direct combination, or pairing; but that acids, bases, and salts, are all alike formed by the substitution of different kinds of atoms for atoms in the water-group; the replacements occurring without disturbing the type of the water-molecule. The water-group may be regarded either as a molecular chain, with hydrogen-atoms at each end, linked by dyadic oxygen, H-O-H; or, as a.compound of the radical hydroxyl H-O-, with hydrogen; and the substitution may be either for one hydrogen-atom, for the two hydrogen-atoms, or for the hydroxyl group. 257. Constitutions of Acids. By comparing the watermolecule with acid molecules, the relations are shown at a glance: Here the hydrogen at one end of the water-chain has been simply replaced by chlorine, and an acid molecule is the result. The chlorine is a simple radical, powerfully electro-negative, which, by replacing hydrogen in the watermolecule, produces an acid. Nitryl (NO,) is a negative compound radical, which also replaces hydrogen in the water-molecule, producing the powerful nitric acid. An acid molecule is therefore one in which a negative radical, simple or compound, is united by oxygen to hydrogen, and it has the general formula R-O-H. 258. Constitution of Bases.-When pure metallic sodium is added to pure water, energetic chemical action ensues, hydrogen is set free, and the water becomes alkaline or basic. If it is evaporated, a white powder is obtained, which is caustic soda, or sodic hydrate. We begin with sodium and water, and get sodic hydrate thus: H-O-H that is, the reaction has consisted simply in the substitution of Na for H in the water-molecule, which has not changed its type. But, as the sodium is a diatomic molecule, it engages two molecules of water, as may be graphically represented : The new molecules have thus exactly the same structure as the old. Had potassium been used, instead of sodium, the reaction would have been the same, with the production of another basic molecule. But sodium and potassium are positive radicals. A basic molecule, therefore, is one in which a positive radical, simple or compound, is united by oxygen to hydrogen, and its general formula is R-O-H. + 259: Constitution of Salts.-If, now, an acid molecule and a basic molecule are brought together, a strong reaction takes place; but, again, it is a substitution that does not impair the molecular type. We get a salt which has + the general formula R-O-R. Thus, an Compounds in which a positive element, or radical, is linked to a negative element or radical by oxygen, or some analogous dyad, are termed salts. |