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 12 parts iron is thus proved to be a kind of matter quite different from either iron or sulphur; the sub

stance 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 12 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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unlike either the ammonia or the hydrogen chloride by the interaction of which it has been formed. If this solid is collected and examined it is found to be characterised by none of the properties which mark off ammonia and hydrogen chloride, respectively, from other kinds of matter.

Ammonia and charcoal when brought together form a mixture; both constituents are easily recognised in the mixture by the same properties as those by which they are recognised when unmixed. Ammonia and hydrogen chloride when brought together form a compound, called ammonium chloride; neither constituent can be recognised in the compound by the properties by which it is recognised when apart from other kinds of matter.

Water boils at 100° when the pressure on the surface of the water is equal to 760 mm. of mercury. At the same pressure alcohol boils at 78°.3. Each of these liquids may be recognised, and differentiated from other substances, by observing its boiling point. A mixture of water and alcohol in about equal parts is placed in a flask, fitted with a thermometer, and connected with a condenser and receiver, as shewn in fig. 9. When the liquid has been heated to boiling the thermometer registers a temperature higher than 78°.3 and lower than 100°; as boiling continues the temperature rises, but a fixed boiling point is not attained. The liquid in the receiver may be proved to be a mixture of water and alcohol, and the portions which distil over soon after boiling begins may be proved to be richer in alcohol than those which distil over after boiling has continued for some time. If the liquid which distils over (the distillate) is collected in a series of flasks, so that each contains that quantity which has come over for a temperatureinterval of (say) 5° or 8o, and if each of these quantities is again distilled, and the distillate for every 3o or 4° is collected in separate vessels, it is possible to effect a rough separation of the original mixture of water and alcohol into two liquids, one of which consists for the most part of water and the other for the most part of alcohol. This separation of the mixture has been effected by making use of a property of each constituent, which property is a characteristic physical property of that constituent when unmixed with other kinds of matter.

Butylene is a colourless liquid, boiling at (about) 3o. Bromine is a dark reddish brown, heavy, strongly smelling, liquid, boiling at (about) 60°. Each of these is a definite kind

of matter characterised by definite properties, of which the boiling point is one. When butylene and bromine are mixed in the ratio of 1 part butylene to 2.86 parts bromine, by weight, a colourless liquid unlike either constituent is produced. The weight of the liquid formed is equal to the sum of the weights of the butylene and bromine. If the liquid is distilled (s. fig. 9) the thermometer registers 160° from the time when boiling begins until the last drops of the liquid

Fig. 9.

have passed over into the receiver; moreover the distillate has the same properties as the liquid before distillation. Butylene and bromine have formed a compound (called butylene bromide), whose properties are very different from those of either constituent, and from which neither constituent can be withdrawn by taking advantage of one of the physical properties, viz. boiling point, belonging to each constituent when uncombined with other substances.

The constituents of a mixture of gases may frequently be 38 separated by making use of the property which gases have of passing through the fine pores of a mass of dry plaster of

Paris. The passage of a gas through such a porous substance as plaster of Paris is called diffusion.

If two graduated glass tubes are each stopped at one end with a thin dry plate of plaster of Paris, if one is then filled with hydrogen and the other

with oxygen, and if both are at once placed in water with the open ends under the water (s. fig. 10), the water will begin to rise in both tubes. As the gases cannot escape at the lower ends of the tubes, they must be passing outwards through the plates of plaster of Paris, and passing outwards more rapidly than air is passing inwards. If the tubes are of the same section and the same length, and if the level of the water in each is observed after a little time, it will be found that the hydrogen has diffused through the porous plate about 4 times quicker than the oxygen.

Fig. 10.

If a similar experiment is made (with proper precautions) with chlorine,-a heavy, yellowish-green, very badly smelling, gas-it will be found that the rate of diffusion of hydrogen is about six times that of chlorine.

Now let there be prepared a mixture of two volumes hydrogen with one volume oxygen. This mixture cannot be distinguished by the eye from its constituents; if a very little of it is placed in a strong glass tube and a flame is brought near a violent explosion occurs. Let the mixture be collected in a gas-holder from which it may be forced at any desired rate by allowing water to enter the gas-holder from a reservoir above. The gas-holder communicates with an arrangement for drying the gases, and this is connected with a long, dry, clay-pipe placed inside a glass tube arranged as shewn in fig. 11. The mixture of oxygen and hydrogen is caused to pass very slowly through the pipe; gas issues at a and b; a tube is filled with the gas issuing at a and another with that issuing at b. The gas collected at b does not burn when a lighted wooden splint is brought near it, but the splint itself burns more brightly; the gas collected at a burns with a slight explosion. The gas issuing at b consists chiefly of oxygen; that issuing at a consists chiefly of hydrogen.

The mixture of hydrogen and oxygen has been partially