THEORIES OF COMBUSTION.

PART XXXII.

AMONG the various operations of chemistry, none acts a more conspicuous part than combustion; and in proportion to its utility in the science, the necessity of thoroughly investigating its nature and mode of action becomes more obvious to the philosophical chemist. It was a conviction of the importance of this inquiry which induced Lavoisier to examine with accuracy the labours of his predecessors, and by this pursuit he was enabled to form a theory of this process, which though not absolutely free from error, was such as well entitled its author to the distinguished rank he holds among philosophers. And as this theory, though capable of improvement, will probably serve for a foundation to every discovery which may be made in this branch of the science, we shall state it here with a view that our readers may be enabled to appreciate its value.

SECT. I.

LAVOISIER'S THEORY OF COMBUSTION.

(SEE APPENDIX NO. 30.)

LAVOISIER'S theory of combustion is founded upon the absorption of oxygen by a combustible body.

We have seen that bodies cannot burn in a limited quantity of air beyond a certain period (p. 260) that a real analysis of this fluid is effected; and that combustion is not confined to the decomposition of the atmosphere only, by absorbing one of its principles; but that it also decomposes oxygen gas, by absorbing, fixing, and rendering more or less solid in the combustible body, the oxygen or basis of the oxygen gas, and disengaging its solvent, the caloric, under the appearance of heat and flame. See p. 228.

Taking this for granted, it follows that combustion is only the play of affinity between oxygen, the matter of heat, and a combustible body.

When an incombustible body, a brick for instance, is heated, it undergoes no change except an augmentation of bulk and temperature, and when left to itself it soon regains its former state. But when a combustible body is heated to a certain degree in the open air, it begins to become on a sudden intensely hot, and at last emits a copious stream of caloric and light to the surrounding bodies. During this emission the burning body gradually wastes away. It either disappears entirely, or its physical properties become totally altered. The principal change it suffers is that of being no longer capable of combustion.

If either of these phenomena, namely, the emission of heat and light, and the waste of substance, be wanting, we do not say that a body is undergoing combustion, or that it is burning.

It follows therefore that every theory of combustion ought to explain the following facts:

1. Why a burning body is consumed, and its individuality destroyed.

2. Why, during the progress of this alteration, heat and light are emitted.

For the elucidation of these objects, Lavoisier's theory has laid down the following laws:

1. Combustion cannot take place without the presence of oxygen, and is more rapid in proportion to the quantity of this agent in contact with the inflamed body.

2. In every act of combustion the oxygen present is consumed.

3. The weight of the products of every body after combustion corresponds with the weight of the body before combustion, plus that of the oxygen consumed.

4. The oxygen absorbed by the combustible body may be recovered from the compound formed, and the weight regained will be equal to the weight which disappeared during the combustion.

5. In every instance of combustion, light, and heat, or fire, are liberated.

6. In a limited quantity of air, only a certain quantity of the combustible body can be burnt.

7. The air, wherein a body has been burnt, is rendered unfit for maintaining combustion, or supporting animal life.

Though every case of combustion requires that light and heat should be evolved, yet this process proceeds very differently in different circumstances: hence the terms ignition, or glowing heat; inflammation, or accension; and detonation, or explosion.

IGNITION

Takes place when the combustible body is not in an aeriform state.

Charcoal, pyrophorus, &c. furnish instances of this kind.

It seems as if the phenomenon of glowing was peculiar to those bodies which require a considerable quantity of oxygen to become converted into the gaseous state.

The disengagement of caloric and light is rendered more evident to the senses in the act of

INFLAMMATION,

Or Accension. Here the combustible substances are more easily converted into an elastic or aeriform state. Flame therefore consists of the inflammable matter in the act of combustion in the gaseous state. When all circumstances are favourable to the complete combustion of the products, the flame is perfect; if this is not the case, part of the combustible body, capable of being converted into the gaseous state, passes through the luminous flame unburnt, and exhibits the appearance of smoke. Soot therefore always indicates an imperfect combustion. Hence a common lamp smokes, an Argand's lamp yields no smoke.

This degree of combustion is very accurately exemplified in the

FLAME OF CANDLES.

When a candle is first lighted, which must be done by the application of actual flame, a degree of heat is given to the wick sufficient to destroy the affinity of its constituent parts; part of the tallow is instantly melted, volatilized, and decomposed; its hydrogen takes fire, and the candle burns. As this is destroyed by combustion another portion melts, rises, and supplies its place, and undergoes a like decomposition. In this way combustion is maintained. The tallow is liquefied as it comes into the vicinity of the flame, and is by the capillary attraction of the wick drawn up to supply the place of what is decomposed; the unmelted tallow by this means forms a kind of cup.

The congeries of capillary tubes which form the wick is black, because the charcoal of the cotton becomes predo

minant, the circumambient air is defended by the flame from oxydating it; it therefore remains for a considerable time in its natural state; but when the wick by the consumption of tallow becomes too long to support itself in a perpendicular position, its upper extremity projects nearly out of the cone of the flame, and there forms a support for an accumulation of soot, which is produced by the imperfect combustion. A candle in this situation affords scarcely one tenth of the light it can give, and tallow candles on this account require continual snuffing.

But if the candle be made of wax, the wick does not long occupy its place in the middle of the flame; its thinness makes it bend on one side when its length is too great for its vertical position; its extremity comes then in contact with the air, and is completely burnt or decomposed, except so much of it as is defended by the continual afflux of the melted wax. This small wick therefore performs the office of snuffing itself. The difficult fusibility of wax enables us to use a thinner wick for it than can be used for tallow, which is more fusible. But wax being a substance which contains much more oxygen than tallow or oil, the light it affords is not so luminous.

DETONATION

Is an instantaneous combustion, accompanied with a loud report; it takes place in general when the compound resulting from the union of two or more bodies occupies less space than the substances did before their union; a vacuum is therefore formed, and the surrounding air rushing in from all sides to fill it up is the cause of the report.

Gun-powder, fulminating gold, silver, and mercury, oxygenated muriate of potash, and various other explosive compounds, are capable of producing very loud detonations, as we shall see hereafter.

Let us now consider the disengagement of light and caloric.