10 c.c. of gas will remain, which may be proved to be hydrogen.

זיינייייר

Fig. 13.

(3) Let there be 50 c.c. hydrogen and 25 c.c. oxygen: no gas will remain.

[It is assumed that every precaution has been taken in measuring the gases, and that all necessary corrections for changes in temperature and pressure have been made.]

The result of these experiments is that hydrogen and oxygen combine to form water in the ratio 2 : 1 by volume, and in this ratio only. Oxygen is 16 times heavier than hydrogen, bulk for bulk; hence 1 volume of oxygen weighs 8 times as much as 2 volumes of hydrogen, measured at the same temperature and pressure; hence the results of these experiments shew that hydrogen and oxygen combine to form water in the ratio 1 : 8 by weight, and in this ratio only.

The composition of several compounds has now been ex- 58 amined quantitatively; in every case it has been found that a specified mass of the compound has been produced by the combination of fixed and invariable masses of two, or more than two, elements. What is stated regarding the quantita

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60

tive composition of these compounds has been found to hold good for all compounds.

Every compound is a definite kind of matter, characterised by certain properties which mark it off from other kinds of matter; every compound is produced by the combination of two or more simpler compounds, or two or more elements ; and these simpler compounds, or these elements, always combine in the same proportion to form the specified compound.

This result of the examination of the quantitative composition of compounds is of fundamental importance in chemistry. It at once enables us to draw a marked distinction between mixtures and compounds. The composition of a mixture is not unalterable; that of a compound is fixed and definite.

This fact regarding the composition of compounds is usually called

The law of constant, or definite, proportions: or the law of fixity of composition.

It

may be stated in various ways; thus,

The proportions in which bodies unite together chemically are definite and constant.

A given chemical compound is always formed by the union of the same elements in the same proportions.

The masses of the constituents of every compound stand in an unalterable proportion to each other, and also to the mass of the compound formed.

The evidence in support of this statement is really the whole body of chemical facts which are at present known. But special experiments have been conducted with the view of testing the law of fixity of composition.

The experiments made by Stas were characterised by the most rigorous and scrupulous accuracy. Stas prepared the compound salammoniac, or ammonium chloride, by four distinct methods; he purified each preparation with the utmost care, and then determined its composition. Ammonium chloride is a compound of the three elements nitrogen, hydrogen, and chlorine. When an aqueous solution of this compound is mixed with a solution of silver in nitric acid, the ammonium chloride is decomposed and the whole of the chlorine formerly combined with nitrogen and hydrogen enters into combination with the silver to form silver chloride. Silver chloride is a heavy white solid; when it is formed as described from ammonium chloride it settles down to the bottom of the vessel in which the experi

ment is conducted, and may be collected, washed, and accurately weighed. In each experiment Stas added 100 parts by weight of pure silver, prepared with the greatest care and weighed with the greatest accuracy, to a solution of ammonium chloride prepared by one or other of four distinct methods; he collected, and most carefully weighed, the silver chloride produced; thus he determined the mass of ammonium chloride which was wholly decomposed by 100 parts of silver. The following numbers are selected from the results obtained by Stas.

100 parts by weight of silver were required to remove, and enter into combination with, all the chlorine from x parts by weight of ammonium chloride :

x = 49-600; 49-599; 49-597; 49-598; 49-593; 49-5974;

49.602; 49.597; 49-592.

Every experiment is attended with certain unavoidable errors. The results obtained by Stas prove beyond doubt that the quantitative composition of the ammonium chloride examined by him was the same, by whatever method that ammonium chloride had been prepared.

We must more fully examine the composition of compounds 61 with the view of learning more of the laws of combination. We found that the composition of magnesia is defined by the statement

The analytical results thus expressed tell that masses of magnesium and oxygen combine to form magnesia in the ratio 60 40 6: 43: 2 = 120 : 80, &c.

:

But two elements often combine to produce two, or more 62 than two, distinct compounds. For instance carbon and oxygen combine to form two compounds. The composition of these oxides of carbon is represented, in parts per 100, thus ;—

But these analytical results may be stated in another form. We may ask, how many parts by weight of oxygen are combined with one part by weight of carbon in each compound?

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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

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.

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 mass of the standard element.

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.