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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 vxygen, 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.

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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,

used, under the microscope, and note the differences in their appearance. Pour a little carbon disulphide on to a very little powdered sulphur, and on to a very little finely divided iron, respectively, and very gently warm each ; the sulphur slowly dissolves in the carbon disulphide, the iron remains unchanged. Pour a little hydrochloric acid on to portions of the iron, and sulphur, used : the iron slowly dissolves and a gas is evolved which can be proved to be hydrogen ; in the case of the sulphur no gas is evolved, nor is there any apparent change.

Now turn to the mixture of iron and sulphur, and determine whether the iron in it is characterised by those properties which we have found belong to iron as a definite kind of matter, and whether the sulphur in it exhibits the properties which belong to sulphur when it is unmixed with other kinds of matter. Experiment proves that the mixture may be separated into iron and sulphur, by shaking it with water, or by dissolving out the sulphur by carbon disulphide, or by holding the iron by a magnet and blowing away the sulphur. Experiment also proves that hydrochloric acid reacts with the mixture to dissolve the iron and leave the sulphur, and that hydrogen is produced in this reaction. Examination of the mixture under the microscope shews the particles of iron, and the particles of sulphur,

Now heat another portion of the mixture of iron and sulphur; it glows throughout: powder the black mass which remains after cooling, and heat it again. Again powder the heated substance, and determine whether iron or sulphur can be detected in it by making use of those properties of iron and sulphur, respectively, which we know characterise these kinds of matter. The appearance and colour of the substance are distinctly different from those either of iron or sulphur; the substance is not separated into iron and sulphur by any one of the three methods (water, magnet, carbon disulphide), each of which separated the mixture of iron and sulphur into its constituents; the substance appears under the microscope to be homogeneous; interaction with hydrochloric acid results in solution of the substance as a whole, and production of a gas which is not hydrogen, but is sulphuretted hydrogen, a body easily distinguished from hydrogen by many prominent properties.

The substance produced by heating a mixture of one part sulphur with 14 parts iron is thus proved to be a kind of matter quite different from either iron or sulphur; the substance produced by mixing sulphur and iron in the ratio 1:13 was proved to possess properties characteristic both of iron and of sulphur. Mixing iron and sulphur has evidently not produced a chemical change : heating the mixture of iron and sulphur has produced a chemical change.

With a mixture of iron and sulphur, we are not especially 34 concerned in chemistry; with the new kind of matter, called iron sulphide, produced by heating the mixture of 1 part sulphur with 14 parts iron, we are especially concerned. Both kinds of matter are Not-Elements: the first is a mixture of different kinds of matter, each of which can be recognised in the mixture by properties which characterise it when it is unmixed with other kinds of matter; the second is a compound formed by the combination of different kinds of matter, none of which can be recognised in the compound by properties which belong to it when uncombined with other kinds of matter.

The class Not-Elements is divided into compounds and mixtures. Chemistry deals with the changes of composition and of properties of compounds. We are at present endeavouring to understand what is meant by the composition of compounds. In order to learn something about the composition of compounds we must gain as clear a notion of the differences between compounds and mixtures as we can at this stage of our progress.

Ammonia is a colourless, volatile, gas, with a very pungent 35 and penetrating smell; charcoal is a black, porous, light, solid : these two kinds of matter may be recognised by these properties. Let a quantity of ammonia be confined in a tube over mercury; and let a few pieces of charcoal (previously heated to remove air from their pores) be passed into the tube. The ammonia is rapidly absorbed by the charcoal, and the mercury rises in the tube.

The appearance of the charcoal is not changed ; only it smells strongly of ammonia. The ammonia is easily removed from the charcoal, with which it is mixed, by warming the charcoal in a small dry flask, and allowing the ammonia to collect in a tube filled with mercury and placed mouth downwards in a vessel of mercury.

Hydrogen chloride is a colourless, volatile, gas, with a very pungent smell

, and most irritating action on the skin. Let a certain volume of ammonia be confined over mercury, and let an equal volume of hydrogen chloride be passed into the vessel ; instantly there is produced a white solid, utterly

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