There are other ways of preparing magnesia, but 100 parts by weight of this compound, however it has been prepared, can always be resolved into 60 parts of magnesium and 40 parts of oxygen.

If the compound of iron and oxygen produced by burning iron in oxygen is analysed it is found that its composition per 100 parts is ;

iron = 72.41

By composition per 100 parts is meant a statement of the mass of each of the elements which by their combination produce 100 parts by weight of the compound (s. par. 46).

An experiment was already described by which the substance potassium chlorate was proved to be a compound of the element oxygen and the less complex compound potassium chloride. 100 parts by weight of potassium chlorate are resolved by heating into 39.13 parts by weight of oxygen and 60.87 parts by weight of potassium chloride; if 200 parts of the chlorate are used, 78.26 parts of oxygen and 121.74 parts of potassium chloride are obtained. Potassium chloride is itself produced by the combination of the two elements potassium and chlorine in the ratio 52.41 to 47.59; i. e. 100 parts of the compound are composed of 52:41 parts of potassium and 47.59 parts of chlorine. The composition of either potassium chlorate or chloride is definite and unchangeable. By whatever method either of these compounds is prepared, it is always composed of the same elements combined in the same proportions.

The composition per 100 parts of the iron sulphide produced 56 by heating together iron and sulphur is ;

In other words the ratio of sulphur to iron is 1:1·75.

Now if a mixture is made of very finely divided sulphur and iron in the ratio 1.: 2, and this mixture is heated, a black solid will be formed characterised by the properties of iron sulphide; but it can be experimentally proved that the substance thus produced is not iron sulphide only, but is a mixture of iron sulphide and iron; and further it can be proved that 2.75 parts by weight of iron sulphide have been formed and that 25 parts of iron remain uncombined with sulphur. Again, if a mixture of 1.25 parts of sulphur with

57

1.75 parts of iron is heated, 2.75 parts of iron sulphide are formed and 25 parts by weight of sulphur remain uncombined with iron. The compound known as iron sulphide is thus shewn to have a definite and fixed composition: a certain mass of sulphur combines with a fixed mass of iron; if there is more iron than this fixed mass, the iron over and above the fixed mass-generally called the excess of iron-does not combine with the sulphur; if there is an excess of sulphur, some of the sulphur does not combine with the iron.

Experiments have been described by which water has been shewn to be a compound of the elements hydrogen and oxygen. If water is a compound, the composition of water must be definite and unchangeable.

A tube of stout glass is divided into a number of equal parts, preferably into cubic centimetres; the divisions are marked on the outside; the tube is closed at one end; two platinum wires pass through the walls of the tube near the closed end, and are bent so that the ends of the wires nearly, but not quite, touch inside the tube (s. fig. 12). The tube is filled with mercury, with proper precautions, and is inverted in a trough of mercury. A small quantity of oxygen is passed into the tube, and the volume of the oxygen is determined; let it be 10 c.c. 20 c.c. of hydrogen are now passed into the tube. The tube is then pressed down on a pad of caoutchouc, and firmly clamped (s. fig. 13). An electric spark from an induction-coil is passed from one platinum wire to the other; combination of the hydrogen and Fig. 12. oxygen occurs instantly, and the inside of the tube is slightly dimmed by the minute quantity of water produced. The tube is now raised slightly from the caoutchouc pad; mercury rushes in and practically fills the tube. It may be proved conclusively that water, and nothing but water, is formed in this experiment.

The result of this experiment shews that 2 volumes of hydrogen combine with 1 volume of oxygen to produce water. Let the experiment be repeated, but with different volumes of hydrogen and oxygen.

(1) Let there be 20 c.c. hydrogen and 20 c.c. oxygen: when the mercury is allowed to rush into the tube 10 c.c. of gas will remain; this gas may be proved to be oxygen.

(2) Let there be 30 c.c. hydrogen and 10 c.c. oxygen:

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

wwww

gas

Fig. 13.

(3) Let there be 50 c.c. hydrogen and 25 c.c. oxygen: no 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: 406 4 = 3 : 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?