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(or more) kinds of matter, each different from, and each weighing less than, the original matter. Potassium chlorate was changed into potassium chloride and oxygen ; water into hydrogen and oxygen ; copper oxide into copper and oxygen : the potassium chloride, or the oxygen, weighed less than the potassium chlorate; the hydrogen, or the oxygen, weighed less than the water; the copper, or the oxygen, weighed less than the copper oxide . But the sum of the masses of the products of each change was equal to the mass of the kind of matter which was changed into these products: the mass of the potassium chloride added to that of the oxygen was equal to the mass of the potassium chlorate changed; the mass of the hydrogen added to that of the oxygen was equal to that of the water; the mass of the oxygen added to that of the copper was equal to that of the copper oxide by the decomposition of which the oxygen and copper were produced.

We have now examined two classes of chemical changes. 30 One class presented to us interactions between two, or more, definite kinds of matter, resulting in the disappearance of the interacting substances, and the production in their place of one, or more, substances very unlike the original kinds of matter. We paid attention to the mass of one of the interacting 'substances, and to the mass of that product of the change which contained the whole of this substance (neglecting other products if other products there were); we found that the parts of the change to which we paid attention consisted in the combination of the whole of one of the interacting substances with either the whole, or a part, of the other substance. In each experiment a certain kind of matter was changed into a different kind of matter, by entering into combination with some substance different from itself. .

The other class of chemical changes presented to us decompositions of one definite kind of matter into two, or more, different substances; the original kind of matter disappeared, and its place was taken by the new substances formed from it. We found that the mass of any one of the new kinds of matter was less than the mass of the matter from which it was derived, but that the sum of the masses of all the new kinds of matter was equal to the mass of the matter which had been changed into these new kinds of matter.

One of the changes considered presented features common to

1 The student should carefully follow the reasoning on which this conclusion was based. (s. par. 28.)

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both classes of changes. Copper oxide interacted with hydrogen; water and copper were produced : but the water was itself shewn to be produced by the union of the reacting hydrogen with oxygen separated from the copper oxide. The mass of the water added to that of the copper was greater than that of the copper oxide; this was because the water formed was composed of the oxygen at first combined with the copper and also the hydrogen which was one of the constituents of the whole changing system. The change of the copper oxide into copper and oxygen was a change belonging to our second class of chemical reactions; but this was accompanied by a change belonging to the other class of reactions, viz. the production of water by the combination of the oxygen separated from the copper with the hydrogen, the presence of which hydrogen in contact with heated copper oxide was the condition under which the separation of copper oxide into copper and oxygen was accomplished.

When various kinds of matter are examined chemically, it is found that they all belong to one or other of two classes, which we may at present call the hydrogen-class and the water-class.

Those kinds of matter which are placed in the hydrogenclass are characterised by this ;—when any one of them is changed into a totally different kind of matter, the mass of this kind of matter is greater than the mass of the substance belonging to the hydrogen-class which was thus changed. These substances have never been changed by separation into unlike parts. They suffer chemical change by combining with another kind, or other kinds, of matter, the test of this combination being that the substance produced differs from, and weighs more than, the original substance.

Those kinds of matter which are placed in the water-class are characterised by this ;-any one of them may be chemically changed by separating it into unlike parts, the test of this separation being the production from a specified mass of the original substance of at least two different kinds of matter the - mass of each of which is less than that of the original substance, while the sum of their masses is equal to that of the original substance. Most, if not all, of the kinds of matter placed in this class may also be chemically changed by combining with some other kind, or kinds, of matter different from themselves, and so producing a new kind of matter weighing more than the original substance.

Those kinds of matter which belong to the hydrogen-class 32 are called Elements: those which belong to the water-class we shall at present call Not-Elements.

About seventy different kinds of matter belong to the class Elements. By no experiments hitherto tried have chemists succeeded in separating any one of these into unlike parts. When the elements are brought into contact with each other or with not-elements; or are subjected to the action of heat, light, electricity, or magnetism, either in the presence or absence of other kinds of matter; or are compressed, or hammered, or drawn into wires, or otherwise mechanically treated; they either remain unchanged into kinds of matter different from themselves, or they combine with other substances and produce new kinds of matter each weighing more than the element from which it has been produced.

In the present state of knowledge then we regard an element as a completely homogeneous kind of matter. We do not assert that an element is completely homogeneous ; that every attempt to separate an element into unlike parts must necessarily fail; but we say that so far as experimental investigation has gone those kinds of matter which are called elements behave as if each were a distinct substance different in kind from all other substances, and were composed of one kind of matter only.

The properties of the elements differ much. The following well-known substances are elements :—iron, lead, tin, silver, gold, copper.

These are all heavy, lustrous, malleable, solids. A few elements are gases at ordinary temperatures and pressures, viz. oxygen, hydrogen, nitrogen, chlorine, fluorine (?): two are liquids under ordinary conditions, viz. bromine and mercury: the others are solids. Some of the elements are found uncombined with other elements in rocks, e.g. carbon, iron, tin, copper, gold, platinum, sulphur ; oxygen and nitrogen form the chief constituents of the atmosphere; hydrogen is sometimes found in volcanic gases. Most of the elements however have been separated from those combinations of them with other elements which are found in rocks, soils, waters, or parts of animal or vegetable organisms.

The colour of many elements is grey to white; a few are yellowish-white, or yellow; one or two are reddish-brown; three are colourless gases.

Some of the elements are very malleable and very ductile; others are very brittle : some melt at very low temperatures; others only at the highest attain

able temperatures, one or two have never been fused. Most of the elements are heavier, some as much as 20 or 22 times heavier, than an equal volume of water; a few are specifically lighter than water. Some are very good conductors of heat and electricity; others are practically nonconductors: most elements are opaque; a few are translucent. Some again very readily react chemically with most of the others to produce new kinds of matter; e.g. compounds of oxygen, of chlorine, of bromine, or of sulphur, with most other elements, are known. On the other hand, some elements e.g. boron and nitrogen, combine directly only with a comparatively small number of other elements.

To state the name of an element is to state the composition of the element: the name is a short symbol for certain properties which characterise that kind of matter to which the name is given, and mark it off from other kinds of matter. So far as we know at present the element is composed of itself; i.e. any quantity of it is not made up of, or formed by the union of, two or more different kinds of matter, but is completely homogeneous.

CHAPTER III.

MIXTURES AND COMPOUNDS.

The different kinds of matter classed together as Not- 33 Elements are composed each of two or more elements. But we must attempt a subdivision of the class not-element : we are not specially concerned with all the members of this class.

If some finely divided iron is intimately mixed with some powdered sulphur, a heavy, greenish-grey, solid is formed. This solid cannot be an element; the method of its preparation precludes this. It is composed of the two distinct kinds of matter, iron and sulphur. It belongs to the class NotElements. But is it one of those not-elements whose properties and composition are studied in chemistry?

Make a very intimate mixture of finely divided iron and sulphur, in the ratio of 1 part sulphur to 14 parts iron by weight. Compare the colour and appearance of this mixture with the colour and appearance of each of its constituents, iron and sulphur: the mixture is neither brownish black, like iron, nor yellow like sulphur; it is not lustrous like iron, nor is its texture that of sulphur. The colour and appearance of the mixture are approximately the mean of the colour and appearance of its constituents.

Place a little finely divided iron in water; the iron sinks : place a little powdered sulphur in water; part of the sulphur floats to the surface. Bring a magnet under a sheet of white paper on which is strewn a little finely divided iron, and blow (with the mouth) along the surface of the paper; a good deal of the iron remains held by the magnet, although the paper is between the iron and the magnet: examine the action of the magnet on powdered sulphur under similar conditions ; the sulphur is entirely blown away, none is held by the magnet. Examine some of the iron, and some of the sulphur,

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