478 CHEMISTRY [CHEMICAL COMPOUNDS. termed salts are formed by the union of oxides of negative with oxides of positive elements. In this article, how ever, the term acid is restricted to compounds containing hydrogen. The acids formed from the oxides of the various elements in the manner above explained all enter into reaction with bases, and with especial readiness with those derived from the highly positive elements, in such a manner that salts are produced, the hydrogen of the acid being displaced by the metal of the base, and water formed ; for example, II 2 SO 4 Sulphuric acid. Ba"0. 2 H 2 = BaS0 4 Barium hydroxide. Barium sulphate. 20H, We may therefore define an acid to be a compound con taining one or more atoms of hydrogen which may be displaced by a metal by the action of a base ; but, as will be evident later on, although applicable to all acids, this definition is of rather too wide a character. The hydrides of fluorine, chlorine, bromine, and iodine also readily enter into reaction with bases, exchanging their hydrogen for other elements ; and as they also possess an acid taste and redden blue litmus, they are universally regarded as acids. The fact that these com pounds are powerful acids is of considerable importance, as showing that it is the association of hydrogen with a negative radicle which renders it subject to displacement by positive elements that is to say, a compound in which hydrogen is associated with a negative radicle will have the properties of an acid, and the more negative the radicle the more readily can the hydrogen be displaced. The com position of all known acids is entirely in accordance with this view; thus, in nitric acid a single atom of hydrogen is associated with the negative compound radicle NO 3 , in sulphuric acid two atoms of hydrogen are combined with the negative compound radicle SO 4 , and in phosphoric acid three atoms of hydrogen are associated with the nega tive compound radicle PO 4 . The oxides of the most negative and most positive elements most readily combine with water to form acids or bases, whilst the oxides of the feebly negative or feebly positive elements either do not unite with water or form extremely unstable combinations. Similarly, the oxides of the most negative and most positire elements unite to gether to form stable salts, much heat being developed by their combination, whereas the oxides of feebly nega tive and feebly positive elements either do not unite or form very unstable salts. The several oxides of an element often differ remarkably in properties, however, one oxide exhibiting acid characters, whilst another has basic pro perties. The oxides of most of the so-called metallic elements are acted upon by acids, such as sulphuric, nitric, and hydrochloric acids, a salt of the metal being formed in all cases in which corresponding salts exist, and the hydro gen of the acid being eliminated in combination with the oxygen of the oxide as water ; for example : MnO + H 2 S0 4 = MnS0 4 + OH 2 . Manganese oxide. Sulphuric acid. Manganese sulphate. Water. If corresponding salts do not exist, however, and action take place, then other products are obtained. Thus, no stable sulphate corresponding with manganese dioxide exists, and when this body is decomposed with sulphuric acid oxygen is evolved : 2Mn0 2 + 2H 2 S0 4 = 2MnSO 4 + 20H 2 + 2 . Manganese dioxide. Sulphuric acid. Manganese sulphate. Water. Oxygen. Actions of this kind invariably occur with the oxides which may be regarded as formed on the type of hydrogen dioxide. Sulphides. Sulphur enters into union with most of the elements forming compounds analogous to the oxides in composition and general properties ; in fact, we may dis tinguish acid and basic sulphides, corresponding to the acid and basic oxides, and salts formed by the union of these two classes of sulphides. With very few exceptions the sulphides are solid bodies. Selenium and tellurium, which are closely allied to sulphur, also form compounds with many elements more or less closely resembling the sul phides, but they are of little importance. Chlorides Bromides Iodides Fluorides. With very fev exceptions compounds of all the elements with chlorine have been obtained, and, especially from a theoretical point of view, the chlorides are a class of bodies of the highest importance. The affinity of bromine and iodine for other elements being much lower than that of chlorine, and their compounds much less stable than the corresponding chlorides, comparatively few bromides and iodides are known. Fluorides of many of the elements have also been obtained. Chlorine unites with many of the elements in two or more proportions, but at most six atoms of chlorine unite with a single atom of another element. The majority of the chlorides are either liquids or solid bodies which may be more or less readily volatilized. Nitrides Phosphides. Nitrogen has but a slight affinity for other elements ; its most important compounds are ammonia, NH 3 , cyanogen, C 2 N 2 , and the oxides of nitrogen. The only elements which combine with it readily are tantalum, titanium, tungsten, and vanadium, and most of its compounds can be prepared only by indirect methods. Phosphorus readily combines with chlorine, bromine, iodine, oxygen, and sulphur, and with most of the metals ; its compounds with the non-metallic elements are of con siderable importance to the chemist ; but its compounds with the metals have been little studied. Scarcely any of the remaining elements form compounds of importance with elements other than those which have already been considered. We now proceed to the description of the elements and their more important compounds, commencing with hydro gen, and then passing to the so-called non-metallic elements, which will be considered in the following order : Hydro gen, oxygen, chlorine, bromine, iodine, fluorine, sulphur, selenium, tellurium, nitrogen, phosphorus, boron, carbon, and silicon ; after which the remaining elements will be briefly described as much as possible in the order of their relationship to each other. Hydrogen is placed at the head of the list, because it is the unit or standard of com parison both for the atomic weights and valencies of the remaining elements, and it is now usual also to refer the densities in the state of gas of all compounds to hydrogen. On account of the number and variety of their compounds with other elements the non-metallic elements are by far the most important, and therefore are naturally considered before the metals. HYDROGEN. Symbol, II ; Atomic wt., 1 ; Molecular vt , 2 ; Valency, . This element was discovered by Cavendish in 17GG, and was called by him inflammable air; the name of Hydrogen is derived from vSwp, water, and yf.vva.ia, to generate, on account of its forming water when burnt. It occurs in the free state in the gases of volcanoes, and by the aid of the spectroscope has been detected in the sun, stars, and nebulae ; it chiefly exists in combination with oxygen as water, and is an important constituent of all vegetable and animal substances.
Hydrogen is obtained by the decomposition of water in