520

Supporters of Combustion.

called supporters of combustion, while the bodies to which they unite have been called combustibles. It is, however, generally ad. mitted that the phenomena of combustion are dependent on the union of the two bodies; and that the so-called supporter is consumed as well as the combustible, and aids in furnishing the light and heat. Thus, copper and sulphur at a high temperature combine with combustion. Which is the supporter, and which the combustible? Both must be regarded as combustible substances, for copper burns in chlorine, and sulphur burns in oxygen. Whether we put phosphorus into the vapour of chlorine, or chlorine into the vapour of phosphorus, the same kind of combustion equally ensues, and the products are similar. During the combustion of phosphorus in oxygen, the intense and sudden burst of light which appears after the phosphorus has entered into the boiling state, arises from the diffusion of its vapour throughout the whole of the oxygen of the vessel, so that there is a combustion of both at every point of contact. Up to this time the light and heat may have appeared to proceed from the solid phosphorus only; but it will now be observed to issue equally from all parts of the vessel containing the oxygen. The oxygen is here as much a combustible as the phosphorus. In fact, the term "combustible" is relative and arbitrary; that body which is for the time in larger quantity, or in the gaseous state, is called the "supporter." Coal-gas burns in oxygen or air only where it can unite with oxygen, and it is therefore called a combustible gas. If we kindle a jet of coal-gas issuing from a bladder, and cause the flame to be projected into a bell-glass of oxygen, it will burn brilliantly. If we fill another bell-glass with coal-gas, ignite it at the mouth, and project into it through the flame, a jet of oxygen, this gas will appear to burn, and in fact does burn, in a jet precisely like the jet of coal-gas, and it will be found to give out the same amount of light and heat, and to give rise to similar products. The oxygen and coal-gas burn only where they meet each other at a high temperature. The oxygen burns in an atmosphere of coal-gas just as certainly as the coal-gas burns in an atmosphere of oxygen. This may be further illustrated by an experiment with an ordinary argand gas-burner. A long chimney-glass should be placed over the burner, and all access of air from below cut off by a cork and a disc of card. If after allowing the coal-gas to issue for a few minutes in order to remove the air, it is ignited at the top of the chimney-glass, a jet of oxygen may be safely propelled downwards through the gas-flame, and the oxygen will appear to burn in the

Heat and Light produced.

521

glass cylinder containing the coal-gas. These facts show that combustion is really a reciprocal phenomenon, each body burning, or, in chemical language, combining with the other body, and during this combination evolving light and heat. The terms combustible and supporter of combustion are, however, convenient for use, provided we understand by them that each substance shares in the process, and that neither is, strictly speaking, passive.

Heat and Light of Combustion.

736. The results of experiments by Despretz show that the heat of combustion in some cases depends, not so much upon the quantity of combustible, as upon the weight of oxygen, consumed. A pound of oxygen, in combining respectively with hydrogen, charcoal, alcohol, and ether, evolved in each case very nearly the same quantity of heat, each raising 29 lbs. of water from 32° to 2120. But with respect to the comparative heating powers of equal weights of different combustibles, Despretz obtained the following results :

This table indicates, not the absolute amount of heat evolved, but the relative heating power of fuels burnt under similar conditions; and it further appears to show that, provided the same weight of oxygen be consumed, whatever may be the nature of the fuel, the same amount of heat will be evolved. In order to produce an intense heat, therefore, the object is not so much to consume the fuel, as to consume the maximum of oxygen with a minimum of fuel. The heating power of the blowpipe and of the blast-furnace, especially of the hot blast (to counteract the cooling effect of the nitrogen associated with oxygen in the air), will now be intelligible

522

Heat from Combustion.

on chemical principles. It is not, however, strictly true that the same weight of oxygen always produces by combustion the same amount of heat. Other experiments performed by Despretz have shown that a pound of oxygen, in combining with iron, tin, and zinc, could heat nearly twice as much water to the same temperature as that which in his table he assigns to hydrogen, carbon, alcohol, and ether; hence, in reference to these metals, oxygen alone cannot be concerned in its production. So with regard to phosphorus: if this substance is burnt slowly, to produce phosphorous acid, a pound of oxygen in combining with it, evolves the same amount of heat as that assigned to carbon and hydrogen; but if the combustion is so intense as to produce phosphoric acid, then the heat evolved is twice as great, resembling that which is given out in the combustion of iron, tin, and zinc. There is another fact which shows that the rule regarding the evolution of heat is not so simple as Despretz had supposed; namely, that when carbon is already combined, as in carbonic oxide, the amount of heat evolved during its combustion and conversion into carbonic acid, is nearly equal to that which would be evolved by the carbon in a separate state, although the latter would require twice the amount of oxygen to convert it into the same product (carbonic acid). (Kane's Elements of Chemistry,' p. 244.) The later researches of Professor Andrews and other chemists have shown that the quantity of heat evolved as a result of the chemical combination of bodies is definite, and that it has a specific relation to the combining number of each substance. With a proper supply of oxygen, or air, a given weight of the substance always produces the same amount of heat.

737. All our ordinary sources of light and heat for domestic and manufacturing purposes are dependent on the combustion of hydrogen and carbon which are found associated in variable proportions in coal, wood, and oil. The following table shows that, according to the experiments of Despretz, hydrogen and carbon, weight for weight, consume the largest amount of oxygen in undergoing perfect combustion; and that hydrogen, in uniting to oxygen, has more than three times the heating power of carbon :

I pound of hydrogen takes
6 pounds of carbon take.

Pounds
of Air.

Pounds of
Oxygen.
8 or

40.

Prop. of Combustible to Oxygen.

1:8

I: 2.6

Hence, by reason of this enormous consumption of oxygen in proportion to the weight of material burned, hydrogen, and bodics

containing it, evolve the greatest amount of heat. Hence, also, in the oxy-hydrogen blowpipe we have one of the highest sources of heat at present known; and as an indirect result of the absorption of this heat by the infusible substance, lime, we obtain a light which rivals that of the sun in intensity and chemical power. Lately, by the construction of a close furnace of lime, and the use of the oxy-hydrogen flame, MM. Deville and Debray have not only been able to volatilize many of the supposed fixed impurities in commercial platinum; but with about 43 cubic feet of oxygen, they have succeeded in melting 25 pounds of platinum in less than three quarters of an hour, and casting it into an ingot in a coke mould. All metals are melted, and many are entirely dissipated in vapour, by the intense heat produced under these circumstances. The lime itself is unaltered by the heat, and acts as a powerful non-conductor, even when not more than an inch in thickness. Lime and magnesia appear hitherto to have resisted fusion, or volatilization as oxides.

In reference to combustion, the improvements made in the use of gas as a source of heat have depended on the admixture of air, or on the free supply of air, by a variety of arrangements; and, in the construction of all furnaces, the adoption of this principle leads to an economy of fuel, the prevention of smoke, and the production of the largest amount of heat. The smokeless flame of a Bunsen's burner, derived from the combustion of mixed air and gas, gives but little light, with an intense heat.

Nature of Flame.

738. It has been elsewhere stated that flame is nothing more than the combustion of volatile or gaseous matter emanating from the heated solid and extending to a certain distance above it. Those bodies only burn with flame which, at the usual burning temperature, are capable of assuming the vaporous or gaseous state. Charcoal and iron burn without flame; their particles are not volatile at the temperature at which they burn. Phos phorus and zinc, on the other hand, are volatile bodies, and there. fore burn with flame. Small particles of each substance are carried up in vapour, are rendered incandescent by the heat of combustion, and burn wherever they meet with the atmospheric oxygen: the more volatile the substance, the greater the amount of flame.

739. The flame of a candle or of gas is hollow, a fact which may be proved by numerous experiments. If a piece of metallic wire gauze be depressed over a flame, this will be seen to form a ring

524

Hollowness of Flame.

or circle of red heat in the metal, dark in the centre, and luminous only at the circumference, where the gaseous particles meet with oxygen. The inflammable matter traverses the meshes of the gauze, but is so cooled by the conducting power of the metal that it ceases to burn above. (See Art. 607.) A piece of stiff paper, suddenly depressed on a spirit-flame to about its centre, presents a carbonized ring corresponding to the circularity of the flame. If a thin platinumwire be stretched across a wide flame of alcohol, it will be heated only at the two points, corresponding to the circumference, where combustion is going on.

740. By allowing a jet of gas to issue from a glass-cylinder in the manner already described, a variety of experiments may be performed to show the hollowness of flame and the comparatively low temperature of the gas or vapour in the interior. An iron wire laid across the cylinder becomes red-hot only at the edges of the chimneyglass. A deal splint will take fire and burn at these points, but be unchanged in the centre. A lighted wax taper fixed on wire, introduced suddenly through the sheet of flame is extinguished in the interior. Gunpowder introduced in a ladle may be held in the inner space within the flame for a long time, and even withdrawn, without exploding. Gun-cotton will not explode under these circumstances if introduced at the end of a copper wire while the coal-gas is freely issuing from the chimney-glass, and the jet is not kindled until after its introduction. That the inner portion of every cone of flame consists of unburnt gas, or combustible vapour comparatively cool, may also be proved by placing within it, the open end of a glass tube, supported by wire, and applying a lighted taper at the other end of the tube which projects out of the flame. The unburnt gas or vapour will be conducted off by the tube, and may be kindled at the end of it, as from an ordinary jet. Thus, then, all inflammable gases and vapours, when unmixed with oxygen, have only a surface combustion, which is defined by the access of oxygen and its contact with the heated gas or vapour. It is different when the burning gas or vapour has been previously mixed with oxygen. Under these circumstances the flame is solid, i.e., combustion is taking place throughout the whole of the mixed gases. Thus, in burning a jet of mixed oxygen and hydrogen, the whole cone of flame is matter in a state of combustion, and the heat is proportionably more intense.

741. Flame in all cases consists of matter ignited to a very high. temperature. Sir H. Davy assigned a white heat (3280°) to ordinary