unite to form a chemical compound, the compound is represented by the symbols of its elements put side by side. Thus HCl represents a compound of hydrogen with chlorine. But we have before seen that whenever hydrogen and chlorine unite, they do so in the proportion of 1 to 35.5. Hence the symbol HCl does not only mean a compound of hydrogen with chlorine, but it also expresses that for every 1 unit of weight of hydrogen there are 35.5 of chlorine. Similarly, the body consisting of 12 carbon and 64 or 2×32 sulphur, is represented by CSS, C2S, CS2, or CS2; the last is the symbol most convenient in use and the one we shall employ. Again, there are two combinations of carbon with oxygen, namely, 12 carbon to 16 oxygen, and 12 carbon to 32 oxygen (or 2×16). The first is represented by CO, the second by CO2. In like manner, the compounds of sulphur with iron (§§ 16, 22, 25) are represented by the symbols FeS, FeS2, Fe,S. A compound symbol, or symbol of a compound, is called the Formula of the compound. § 45. Further, a numeral placed in the same line as the formula implies a multiplication by the numeral of all the subsequent symbols, as far as some mark, comma or bracket, or, wanting this, to the end of the formula. Thus the body alcohol contains and consists of carbon, hydrogen, and oxygen. Its simplest formula is CHO. It may also be written 02CH,, or C ̧ ̧ÃO, and in a variety of other ways. The first of these formulæ, CHO, which merely expresses the simple numerical relation of the equivalents, is called the Empirical formula; the others, which pretend to show something of the arrangement of the elements, knowledge of which may be gathered from the synthesis or analysis of alcohol, are called Rational formulæ. § 46. When different bodies are to be represented as existing together for any time without chemical change, as when two bodies are placed together in order that they may react upon one another, their symbols or formulæ are joined by the sign +. So, when a chemical action, either synthesis or analysis, gives rise simultaneously to different bodies, the symbols or formulæ of the products are also connected by the sign +. The chemical change which takes place is shown by an equation whose sides are respectively the symbols or formulæ of the bodies before and after the chemical change. Thus : : § 47. Since matter cannot be destroyed nor elements transformed into one another, it follows that there must always be the same number of equivalents of each element after as before the chemical change has taken place. § 48. It is sometimes convenient to employ the sign pressing a decomposition. Thus CH2O HO = CO + CO,. Such an equation is of course equivalent to 2 4 in ex But the first equation shows that the withdrawal of H2O from CH2O, gives rise to CO and CO2, which is the fact; the second might imply that C,H,O, were made out of H2O and CO and CO,, which may be the case, but has not yet been done. 2 4 2 We shall henceforth make free use of the symbols of the elements instead of their names; and the student should at once familiarize himself with them. They are all found in the Table, pages 72, 73. § 49. Nomenclature.-The elements divide themselves naturally into two classes, the metals and the non-metallic elements. The former is by far the most numerous. The principal characteristics of a metal with regard to appearance are its opacity and the great perfection with which a smooth surface of it reflects light. These properties combined, constitute what is called the metallic lustre. Metals in the solid state have different densities, varying from that of the heaviest known body (an alloy of osmium and iridium, sp. gr.=22.6) to a density less than that of water (lithium =0·594, being the lightest known solid). All metals may be melted; but mercury is the only unmixed one known in the liquid state at ordinary temperatures. Several (probably all) metals may be converted by heat into vapour, and in that state appear as colourless transparent gases, exactly resembling permanent or hitherto uncondensed gases; so that if some of the permanent gases could be liquefied or solidified, they might be found to have the metallic lustre. One great character of the metals is that they may all, in the liquid state, be mixed with one another, forming alloys; but they all refuse to dissolve in the non-metallic elements or their compounds, unless they first unite chemically with one or more of them. § 50. Of the remainder of the elements, the non-metallic ones, a few, O, H, N, and perhaps Fl*, are permanent gases; that is, they cannot be condensed by cold or pressure into liquids or solids. One, chlorine, Cl, is under ordinary circumstances a gas, but can be condensed to a liquid. Bromine, Br, is usually a liquid, but is easily converted into gas by gaining heat, and into a solid by losing it. The rest are more or less easily liquefiable and vaporizable solids, with the exception of carbon (C), boron (Bo), and silicon (Si), which cannot be volatilized or melted when alone. There are certain elements, such as arsenic (As), which resemble the metals proper in metallic lustre, insolubility, &c., but which are like the non-metallic elements in several of their chemical properties. § 51. Beyond the metals proper, whose chemical names all end in um, the only systematic termination borne by the elements which shows similarity of property is ine, as in chlorine, bromine, &c. The elements oxygen, hydrogen, nitrogen, which all end in gen, have little chemical relationship, though as gases they have some physical similarity. § 52. When two elements unite together chemically, a compound name is given to the resulting body, a name made up of the names of the elements which the body contains. Thus CO is called the oxide of carbon, FeS the sulphide of iron, and so on. When two elements unite with one another in more than one * The properties of elementary fluorine are not yet fully established. proportion, a corresponding variation is made in the name, Latin or Greek numerals being often employed. Thus is called oxide of carbon (or carbonic oxide). binoxide of carbon (or carbonic acid). oxide of iron. sesquioxide of iron (1 to 14). 2 four-thirds oxide of iron. 3 CS 2 When the compound body consists of a metal or hydrogen united with a non-metallic body, it is usual to write the symbol for the metal towards the left hand, and to attach the ending ide to the name of the non-metallic element. This is a matter of choice, and is not invariably adopted. Thus FeO, oxide of iron, might be written OFe, ferride of oxygen. CS, bisulphide of carbon, might be written S,C, carbide of sulphur. § 53. The compound of two elements which consists of equal numbers of their equivalents is generally distinguished by the prefix prot(o) or mon(o). Thus FeO is the protoxide of iron. § 54. If a metal unites with a non-metallic element in such a way that there is more than one equivalent of the metal for every equivalent of the other element, the prefixes di or sub are often used. Thus CuO is the oxide of copper or protoxide of copper. CuO is the suboxide of copper or dioxide of copper. § 55. Sometimes the prefixes hyper or per are used to denote an excess of the non-metallic element, and the prefix hypo a deficiency of it beyond a certain quantity. § 56. The terms used by different chemists for the same substance are often different according to the different views they take of the rational composition of it (see § 45); and in very many cases the discovery of new compounds has made the names given to the previously known ones inconsequent. The nomenclature bears the marks of the history of the science, and is a compromise between new and old views. The names of the few bodies which we shall have to consider will be easily acquired as we proceed. CHAPTER IV. ACIDS, BASES, and Salts. $57. Many compound bodies, consisting of two or more elements, hold one of those elements so loosely that it can be withdrawn and replaced by another element, the remaining elements of the original compound resting undisturbed. The newly introduced element can also be replaced by another, and so on. This is especially the case with a large class of substances containing hydrogen, and which are called acids. Such substances usually have, when soluble, a peculiar sour taste, similar to that of lemonjuice or vinegar. They have also the power of changing the colour of certain vegetable colours, turning red such as are blue. The term acid is also frequently, but somewhat improperly, applied to bodies which do not contain hydrogen or other replaceable element, but which, when united with, or even in presence of, water, cause the hydrogen of the water to be replaceable by a metal or similar body. Examples of such bodies are CO,, SO2, &c. * We shall find that a group or compound of non-metallic elements may play the same part chemically as a metal. |