36

37

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 8°, and if each of these quantities is again distilled, and the distillate for every 3o or 4o 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.

H

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

separated into its constituents by making use of a physical property of these gases called rate of diffusion, which property

a

Fig. 11.

is a characteristic mark of each gas when unmixed with other substances.

If now a mixture is made of equal volumes of hydrogen 39 and chlorine, and this mixture is exposed to diffused sunlight for some time, a new gas will be formed; the new gas is colourless; chlorine is yellow, hydrogen is colourless-: the new gas fumes much in the air-neither chlorine nor hydrogen fumes in air—; it has an intensely acrid smell, quite different from the smell of chlorine. This gas is called hydrogen chloride. The weight of hydrogen chloride formed is equal to the sum of the weights of the hydrogen and chlorine which have combined to form it.

If hydrogen chloride is passed through, a dry clay-pipe under conditions similar to those already described it can be proved that no separation into hydrogen and chlorine has occurred, but that the gas which issues at a is identical with that which issues at b, and that both are hydrogen chloride.

The compound of hydrogen and chlorine (hydrogen chloride) cannot be separated into its constituents by making use of a certain physical property-viz. rate of diffusion-which belongs to, and characterises, each of its constituents when these are uncombined with other kinds of matter.

The results of experiments such as those considered in 40 pars. 33 to 39 shew that the substances comprised in the class Not-Elements may be divided into two groups; compounds and mixtures.

41

Each constituent of a mixture retains in the mixture the properties which characterise it when unmixed with other substances: the properties of the mixture are, broadly, the sum of the properties of the constituents. No constituent of a compound retains in the compound the properties which characterise it when separated from other substances: the properties of a compound are not the sum of the properties of the constituents; the compound is a definite kind of matter, as distinct from each of its constituents as these are from one another, and yet formed by the combination of these constituents.

To say of a mixture, that it contains the bodies by mixing which it has been produced, is to use an expression which conveys a correct notion of the relations of the properties of the mixture to those of its constituents. But it is not so correct to say that a compound contains each of those kinds of matter by the interaction of which it has been formed. Thus, a mixture of iron and sulphur contains iron and contains sulphur; inasmuch as, not only is the mass of the mixture the sum of the masses of the mixed iron and sulphur, but the properties of the mixture are also the properties of iron added to those of sulphur. The mass of a compound of iron and sulphur is certainly the sum of the masses of the iron and sulphur which have combined to form it; but the properties of the compound are quite distinct from the properties by which iron or sulphur is marked off from other kinds of matter.

The formation of a mixture is a physical process.

The properties of every mixture probably differ slightly from the sum of the properties of its constituents; some change occurs in the formation of the mixture; nevertheless the properties of each kind of matter in the mixture are so slightly modified by the presence of the other kinds of matter that it is always possible, and generally easy, to recognise each of these kinds of matter by some, or all, of the properties which distinctly mark it off from other kinds of matter.

The formation of a compound is a chemical process.

The properties of each of those kinds of matter which combine are so largely modified by the presence of the other combining substances that it is impossible to recognise any of them by the properties which belong to it when uncombined. Iron sulphide is as distinct and definite a kind of matter as iron or sulphur; ammonium chloride is as distinct and definite