75 These numbers represent parts by weight of each element which combine with 8 parts by weight of oxygen, 16 of sulphur, or 35.5 of chlorine; in other words these numbers represent the parts by weight of each element which combine with one combining weight of oxygen, or sulphur, or chlorine. The conception of combining weight may be extended to all the elements. The combining weight of an element which forms a compound with hydrogen must be regarded by us at present as a number expressing the mass of the element which combines with unit mass of hydrogen. The combining weight of an element which does not form a compound with hydrogen we shall for the present regard as the mass of that element which combines with one combining weight of oxygen, or of sulphur, or of chlorine; i.e. with that mass of oxygen, sulphur, or chlorine, which combines with unit mass of hydrogen, i.e. with 8 parts by weight of oxygen, 16 of sulphur, or 35.5 of chlorine. The laws of multiple, and reciprocal, proportions may now be put into one statement. The elements combine in the ratios of their combining weights, or in ratios which bear a simple relation to these. To illustrate this mode of expressing the laws of multiple and reciprocal proportions, let us tabulate (1) the combining weights of several elements, (2) the compositions of several compounds of these elements stated as so many combining weights of each element. Combining weights of some elements. Composition of some compounds; stated as number of combining weights of each element. Oxides of nitrogen. c. ws. of oxygen: c. ws. of nitrogen = 1 : 1 in compound a, 1:3 in compound b, 2:3 in compound c, 4:3 in compound d. Oxides of chromium. c. ws. of oxygen: c. ws. of chromium in compound a, 8:9 in b, 1:1 in c, 4:3 in d, 2:1 in e. Chlorides of antimony. c. ws. of chlorine: c. ws. of antimony=1:1 in compound a, 5:3 in b. Bromides of tin. c. ws. of bromine : c. ws. of tin compound a, 1:1 in b. c. ws. of iodine: c. ws. of mercury = 1:2 in = 1:2 in Iodides of mercury. compound a, 1:1 in b. Sulphides of copper. c. ws. of sulphur : c. ws. of copper = 1:2 in compound a, 1:1 in b. r The composition of all compounds may be stated in this way. Let us use a symbol to represent one combining weight of an element. Let N represent one combining weight of nitrogen; N,, two c. ws. of nitrogen; N,, three c. ws. of nitrogen; generally N x c. ws. of nitrogen: let O represent one c. w. of oxygen; x c. ws. of oxygen: Cr, one c. w. of chromium: Sb, one c. w. of antimony: Sn, one c. w. of tin: Hg, one c. w. of mercury: Cu, one c. w. of copper: Cl, one c. w. of chlorine: Br, one c. w. of bromine: I, one c. w. of iodine and S, one c. w. of sulphur. Then the compositions of the above compounds may be represented thus ; Oxides of nitrogen. ON, ON, ON, ON 2 ვი Oxides of chromium. O'Cr, OC, OC, Cr, O.Cr. Bromides of tin. BrSn,, BrSn. Sulphides of copper. SCu,, SCu. CHAPTER VI. SYMBOLS AND FORMULAE. 76 Ir is customary to express the composition of compounds in a kind of shorthand by a method the principle of which is the same as that we are at present illustrating. A symbol is given to each element; this symbol is formed either of the first letter, or of the first and some other letter, of the name of the element. When the names of several elements begin with the same letter that element which has been longest known and best studied generally gets a symbol formed of the first letter only; but there is no universally applicable rule. Some of the symbols are derived from the names by which the elements were known to the ancients or in the middle ages. The symbols of two elements, potassium (K), and sodium (Na), are derived from the names kalium and natrium by which these elements are known to German chemists. The symbol W is given to the element tungsten, it is derived from the name (Wolfram) of the mineral from which tungsten was first obtained. It is of the utmost importance to remember that each of these symbols represents a definite mass of the element; it represents either one, two, three, four, five, or six, combining weights, as we are at present using the term combining weight, of the element. The following table gives the names and symbols of the elements. That collocation of symbols which expresses the composition of a compound is called the formula of that compound. The formulae BaO, B,O,, Cr,Cl, HI, tell, that barium and oxygen combine to form barium oxide in the ratio 137:16 by weight, that boron and oxygen combine in the ratio 22:48 (= 11 × 2:16 × 3), that chromium and chlorine combine in the ratio 104·4 : 213 (= 52·2 × 2 : 35·5 × 6), and that hydrogen and iodine combine in the ratio 1: 127. Or, the facts concerning composition which these formulae express may be thus stated; 153 parts by weight of barium oxide are formed by the combination of 137 parts by weight of barium with 16 parts by weight of oxygen; 70 parts by weight of boron oxide are formed by the combination of 22 parts by weight of boron with 48 parts by weight of oxygen; 317.4 parts of chromium chloride are produced by the combination of 104·4 parts of chromium with 213 parts of chlorine; 128 parts of hydrogen iodide are formed by the union of 1 part of hydrogen with 127 parts of iodine. The numbers in the third column of the preceding table are sometimes called the combining weights of the elements. We have already given a meaning to the term combining weight (s. par. 74). If that meaning is adopted, the mass of an element expressed by its symbol is seldom the same as the value obtained for the combining weight of that element; but when it is not the same, it is a simple multiple of the combining weight. We are not yet in a position to go fully into this matter of combining weights. We have already used the expression combining weight to mean, that mass of an element which combines with unit mass of hydrogen, or, in the cases of elements which do not combine with hydrogen, that mass which combines with 8 parts by weight of oxygen, or 16 of sulphur, or 35.5 of chlorine. But when we come to apply this definition we meet with many difficulties. Thus, nitrogen and phosphorus each form one compound with hydrogen; nitrogen forms 5 compounds, and phosphorus 2 compounds, with oxygen. From the composition of each of these compounds a value may be deduced for the combining weight of nitrogen, or for that of phosphorus. Similarly iron forms 3 compounds with oxygen, and 2 with chlorine; from the composition of these, values are found for the combining weight of iron. The values are these. |