Elementary Chemistry

Here we see that the mass of oxygen which has combined with unit mass of carbon to form compound II. is exactly double that which has combined with unit mass of carbon to form compound I.

Carbon and hydrogen combine to form many compounds; let us select four of these and state their compositions in parts of carbon and hydrogen per 100 parts of each compound. The results are as follows ;

Compound I. is called acetylene, II. is called ethylene, III. ethane, and IV. methane.

If these results are treated as we treated the analyses of

the oxides of carbon, we find that

1 part by weight of carbon is combined with

083 parts by weight of hydrogen in compound I.

Five compounds of the two elements nitrogen and oxygen are known; if the composition of each is determined and is stated as parts by weight of oxygen combined with 1 part by weight of nitrogen, we have this result;

Compound I.

57 parts by weight of oxygen combined with 1 part by weight of nitrogen.

II. 2 × 57 oxygen with 1 of nitrogen.

III. 3 × 57

By examining the composition of series of compounds of the same two elements and tabulating the results as we have done for the compounds of carbon and oxygen, carbon and hydrogen, and nitrogen and oxygen, we arrive at the second law of chemical combination, which is generally known as

The law of multiple proportions. This law may be stated thus;

When one element combines with another in several proportions, these proportions bear a simple relation to one another. Or, better, thus;

When two elements combine to form more than one compound, the masses of one of the elements which combine with a constant mass of the other element bear a simple relation to each other.

In the cases considered we have kept the mass of one 64 of the combining elements constant and have taken this mass as unity, and we have found that the masses of the other element which combine with this constant mass are all whole multiples of one quantity, viz. the smallest mass of the other element which combines with the constant standard element.

mass of the

The relation between the combining masses of the second element is evidently in these cases a very simple one. But this relation is not always so simple. Thus iron and oxygen combine to form three distinct oxides of iron; the ratio of the quantities by weight of oxygen which severally combine with one part by weight of iron is

Again lead and oxygen combine to form four distinct compounds; the masses of oxygen which combine with 1 part by weight of lead are, severally,

077, 103, 116, and 154.

The ratio of these is

1: 1·33: 1·5 : 2.

The law of multiple proportions confirms, and also goes 65 beyond, the law of constant proportions. The latter law is the statement of the fundamental fact that the composition of every compound is definite and unchangeable; the former law generalises the composition of series of compounds of pairs of elements, and states that such compounds are produced by the combination, with a constant mass of one element, of masses of the other element which are simple multiples of the smallest of these masses.

We are now ready to advance a step further, and to con- 66 sider the composition of compounds of one element with various other elements, and compounds of these other elements

with each other.

We shall begin by considering compounds of the element potassium with (1) chlorine, (2) iodine; and then (3) the compound of chlorine with iodine.

The percentage compositions of these compounds are ;—

Let us find the masses of chlorine and iodine which severally combine with the same mass of potassium: this may be done by finding, (1) the mass of iodine combined with 52.4 of potassium, or (2) the mass of chlorine combined with 23.6 of potassium ;

(1) 23.6: 52·4=76·4 : x.

(2) 52.4 23.6 = 47·6 : x'.

We shall choose (2); x' = 21·4.

That is, with 23-6 parts by weight of potassium there combine, (1) 76-4 parts of iodine to form potassium iodide, (2) 214 parts of chlorine to form potassium chloride.

Now let us turn to the compound of chlorine and iodine. Let us ask; what is the mass of chlorine which is combined with 76.4 parts of iodine?

We have now this result;—

23.6 parts by weight of potassium combine with

76.4 parts by weight of iodine. 21.4

76.4 parts by weight of iodine combine with

chlorine.

21.4 parts by weight of chlorine.

Or, stated more generally, the masses of chlorine and iodine which severally combine with a constant mass of potassium are also the masses of chlorine and iodine which combine with each other.

If another element is used instead of potassium, will a similar result be obtained? Chlorine combines with hydrogen to form hydrogen chloride; iodine also combines with hydrogen

to form hydrogen iodide. If the compositions of these compounds are tabulated we have the following results ;

Treating these results as before, we find that 79 parts by weight of hydrogen combine with

99.21 parts by weight of iodine.

Then we inquire; how much iodine combines with 27.8 chlorine? The answer to this is found from the composition of iodine chloride; it is 99.21. So that we complete the foregoing statement by adding

99.21 parts by weight of iodine combine with

27-8 parts by weight of chlorine.

Or, stated more generally, the masses of chlorine and iodine which severally combine with a constant mass of hydrogen are also the masses of chlorine and iodine which combine with each other.

Hydrogen combines with oxygen to form water; hydrogen 68 also combines with sulphur to form hydrogen sulphide; oxygen combines with sulphur to form oxide of sulphur. Let us examine the compositions of these compounds. We need not state the composition of each in parts per 100; let it suffice to state the results thus

1 part by weight of hydrogen combines with

8 parts by weight of oxygen. 16 sulphur.

Then we inquire; how many parts by weight of sulphur combine with 8 of oxygen? Experiment tells that 8 parts by weight of sulphur combine with 8 parts by weight of oxygen. We have then this result

1 part by weight of hydrogen combines with

8 parts by weight of oxygen.