[Chap. 11. the atmosphere; so that oxygen not only performs for us an infinite number of valuable and important offices, but appears to be one of the grand agents of decomposition and destruction ".

the oil, imparting to it the property of drying quickly. Oil thus prepared is called drying oil.

a All the oxides with which we are acquainted may be thus classed. I. That arising from the combustion of hydrogen, by which water, the oxide of hydrogen, is formed. II. Metallic oxides. III. Acidifiable oxides; or such as by an addition of oxygen pass from the state of oxides to that of acids. The oxides of phosphorus, sulphur and nitrogen, with four of the me tallic oxides, admit of this change. IV. Various combinations of carbon and hydrogen in the animal and vegetable kingdoms. To this class common charcoal, oils, gums, resins, wax, sugar, yeast, soap, &c. belong.

All organized beings, whether vegetable or animal, possess the materials of which they are composed only for a limited time life itself is a boon which is only lent, to serve the purposes of infinite beneficence. At the proper period, oxygen, or some other powerful agent, effects the decomposition of the cu rious fabric, and sets all the elementary particles at liberty, to form other equally perfect and complicated existencies *, "Which thus alternating with death, fulfil

The silent mandates of the ALMIGHTY's will;
Whose hand, unseen, the works of nature dooms,
By laws unknown,-WHO GIVES AND WHO RESUMES."

• See Additional Notes, No. 26.

DARWIN,

CHAPTER XII.

OF COMBUSTION.

WHAT is combustion?

a

Combustion is a process by which certain substances decompose oxygen gas, absorb its base b, and suffer its caloric to escape in the state of sensible heat.

Are all substances capable of being burnt?

No: some substances are combustible, others incombustible.

a Lavoisier and other French chemists defined combustion to be the combination of any body with oxygen. This definition, however, has very properly been objected to; for there are many instances of oxygen combining with a body without producing combustion. Indeed, this union is sometimes effected when no combustion can possibly take place. Oxygen often combines with bodies without any sensible extrication of heat or light; but we never, in common language, give the name of combustion to any operation in which heat and light are not liberated.

To render the explanation of this phænomenon more perspicuous, it may be said that oxygen exists in the state of gas in atmospheric air; that when a combustible is heated to a certain degree, it possesses such an attraction for oxygen, that it absorbs it from the air, and fixes it in a solid form; while the light and caloric, the solvents which gave the oxygen its aëriform shape, escape and diffuse themselves among the surround. ing bodies. See Additional Notes, No. 36 and 38.

The term combustible is applied to every body that is capable of being burnt in atmospheric air, or in oxygen gas, and, consequently, of uniting with oxygen.

How are combustible bodies classed by chemists? Into simple combustibles, compound combustibles, and combustible oxides a.

What do you mean by SIMPLE combustibles? Those combustible substances that have resisted every attempt to decompose them, are called simple combustibles ".

Endeavour to enumerate the simple combustibles. The simple combustibles with which we are acquainted are hydrogen, sulphur, phosphorus, carbon, and all the metals, except gold, silver, and mercury.

• The combustible oxides consist of combinations of the combustible bodies, which have not undergone combustion, or their compounds, with oxygen. This class of bodies is very numerous, as it includes the greater part of animal and vegetable substances. The great combustibility of combustible oxides is probably owing to the weaker affinity by which their particles are united. Hence, they are more easily separated than homogeneous particles, and of course combine more readily with oxygen. Those simple combustibles which melt easily, or which are in the state of elastic fluids, are also very combustible, because the cohesion between their particles is easily overcome. See Dr. Thomson's paper in Nicholson's Journal for 1802.

For an account of the nature and properties of the simple combustibles, see Chap. ix. page 251.

Although sulphur and phosphorus are usually classed with the simple combustibles, from some late experiments of Sir Humphry Davy's we have reason to believe that these are not strictly simple substances. See Phil. Transactions for 1809.

We may be satisfied that metals are really combustible, by repeating the following simple and beautiful experiment of Dr. Ingenhousz:-" Tivist a small iron wire into the form of a corkscrew, by rolling it round a small stick; fix one end of it into a cork (which will fit a glass jar previously filled with oxygen gas), and lap round the other end a small bit of cotton thread dipped in melted tallow. Set fire to the cotton, and immediately plunge the whole into the jar of oxygen gas,

What are COMPOUND combustibles?

Compound combustibles are all such as are formed by the union of two or more of the simple combustibles". Common coal is an instance of this combination b.

What substances are there which are incombustible?

We are acquainted with thirteen incombustible. substances; viz., nitrogen, the three alkalies, and the nine earths d.

What is the nature of combustion ?

Combustion appears to be a double decompo

The wire will take fire from the cotton, and burn with great brilliancy, throwing out very vivid sparks in all directions, During the combustion, the iron combines with the oxygen in the jar, and is converted into an oxide, with an augmentation in weight of 35 per cent." Mr. Accum says, that a thick piece of iron or steel, such as a file, may be burned in oxygen gas, if it be made very sharp-pointed, and a small piece of wood be stuck upon its extremity, and set fire to previous to its being immersed in the gas. The method of suspending the metal in the jar may be seen in plate 4, fig. 17, of Lavoisier's Elements of Chemistry.

a The compound combustibles are arranged by Dr. Thomson under the five following heads: 1st, sulphurets; 2d, phosphurets; 3d, carburets; 4th, alloys; 5th, sulphuretted, phosphuretted, and carburetted hydrogen.

b Hydrogen and carbon, intimately united in the capillary tubes of vegetables, form bitumens, oils, and resins, which are compound combustibles. See Additional Notes, No. 39.

Perhaps nitrogen might be called a combustible body, as by means of electricity it combines with the base of oxygen gas, and forms nitrous acid.

Some of the alkalies and earths possess certain properties in common with combustibles, and are capable of exhibiting phanomena somewhat analogous to combustion. These have been described under the title of semi-combustion; but for particulars I must refer the reader to Dr. Thomson's Theory of Combustion, in Nicholson's Journal for 1802.

To the old chemists the process of combustion was quite

sition, in which the combustible and the supporter of combustion divide themselves each into two portions, which combine in pairs; the one forming the product, the other the fire which escapes b.

What do you mean by SUPPORTERS of combustion?

The substances which are called supporters of

inexplicable; its nature, indeed, was not at all understood till within these thirty years. It is now known to be merely a play of affinities between oxygen, light, caloric, and the base of the combustible body; so that nothing is really lost, but new modifications of the same ingredients take place.

a Dr. Thomson's account of combustion will convey to the reader a clear idea of this natural phænomenon.—“ When a store, or brick, is heated, it undergoes no change except an augmentation of temperature, and when left to itself it soon cools again, and becomes as at first. But with combustible bodies the case is very different. When heated to a certain degree in the open air, they suddenly become much hotter of themselves, continue for a considerable time intensely hot, sending out a copious stream of caloric and light. This emission after a certain period begins to dimini-h; and at last ceases altogether. The combustible body has now undergone a most complete change; it is converted into a substance possessing very different properties, and no longer capable of combustion. The product is incombustible, because its base being already saturated with oxygen cannot combine with any more. M. Lavoisier fully established the existence of this general law-that ‘in every case of combustion oxygen combines with the burning body. Oxygen does not combine with a combustible body till its temperature is raised; but, when a combustible body is raised to a certain temperature, it begins to combine with the oxygen of the atmosphere, and this oxygen during its combination lets go the caloric and light with which it was combined while in its gaseous state." Dr. Thomson's System of Chemistry, vol. i. 417.

b The component parts of the oxygen which is furnished by the supporters of combustion are two; viz. oxygen and caloric: the component parts of all combustibles are likewise two; viz. the base and light. If the two first are called No. 1 and 2, and the two latter No. 3 and 4, the product of combustion will