LECTURE XXXI.

CHEMISTRY.

COMBUSTIBLE SUBSTANCES.

COMBUSTIBLES are substances capable of combustion, which process has been explained in a preceding lecture. You will remember, that about one-third of the air of our atmosphere is oxygen gas, or air. This part of the air is always destroyed by the process of combustion, as is proved by letting a candle burn under a receiver till it goes out. The assumed explanation of the phænomenon is, that the combustible body, being in part decomposed by the action of heat, unites with the oxygen of the air, and together they form a third substance (as hydrogen or inflammable gas and oxygen form water*), and by this condensation of the oxygen, a large quantity of caloric escapes, and becomes sensible, so as to produce heat, and frequently flame. But, perhaps, our knowledge will not at present allow us to say more decidedly of combustion, than that it is the effect of intense chemical action.

* If a receiver, properly cooled, is placed over one of Argand's lamps, a large quantity of water will be found

in it.

As the contact and succession of fresh air is necessary to the combustion of bodies, we find that it takes place always on the surface. Thus a piece of charcoal is only consumed on the surface; for if we break it, we find it still the same in the middle. Thus, spirit of wine burns at its external surface, for a cotton wick remains without change.

Of combustibles, some burn without, and some with flame.

The first are, charcoal of various kinds, metallic bodies, if they may be classed as such, &c. The second are very numerous, as sulphur, oils, spirit of wine, &c. The proper distinction between these classes is, that the one is fixed, and the other volatile. In the fixed body, the consumption can only happen on the surface, while those that are volatile are converted into vapour in a heat below that necessary for their inflammation; and consequently the vapour only can be heated to a proper degree and set on fire. Flame is only a succession of burning vapour, therefore it is always in some degree conical; for it is consumed as it ascends, therefore diminishes from a broad basis, and terminates in a point. This form is most observable in the flame of spirit of wine. The vapour, as it rises, is not wholly consumed; the reason is, the air has not free access to the centre of the column of vapour. The combustion, therefore, takes place where the air has freest access, viz. on the

surface. If a ball of sulphur is suddenly plunged into the flame, it will appear that a considerable part of the vapour is not on fire. The flame is a hollow cone, surrounding an entire cone of vapour.

Flame vibrates, because the vapour is thrown out irregularly from the body, and because, being an elastic fluid, the smallest disturbance in the air makes the two springs, i. e. of the air and of the vapour, act on each other, and produce expansion and contraction.

Soot is a part of the burning vapour which has not been so much inflamed as the rest. In large flames, the surface on which the air acts is less in proportion to the quantity of vapour than in small flames; hence, proportionally, more smoke and soot is formed from large than small flames.

Combustibles are judiciously divided into simple and compound. 1. The simple are hydrogen, phosphorus, sulphur, and carbon. Dr. Thomson observes that the metals might be included, but the greater number of their properties are so different from the four bodies just mentioned, that it is more proper to consider them as a distinct class of bodies. This judicious philosopher adds:-" All our classifications are in fact artificial; Nature does not know them, and will not submit to them. They are useful, however, as they enable us to learn a science sooner, and to remember it better: but if we mean to

derive these advantages from them, we must renounce a rigid adherence to arbitrary definitions which nature disclaims.

1st. Of HYDROGEN it was necessary to treat so amply as a simple substance, that scarcely any thing remains to be said of its combustible qualities. If hydrogen gas is mixed in a receiver with oxygen gas, in the proportion of fifteen parts of the former to seventy-five of the latter, and an electric spark is made to pass through them by means of a wire inserted in the receiver, they will explode, and pure water will be formed. This experiment is generally made in a tub of mercury, with the pneumatic apparatus, as described in Lect. 25. See Pl. VI. fig. 19.

Philosophical fire-works are also made by means of different mixtures of this gas with other aërial fluids. Bladders, with glass tubes and brass cocks affixed to their necks, are filled with the different modifications of this gas; and when the cocks are turned, the inflammable gas issues out, and may be inflamed by a candle. -Pure hydrogen gas affords a white flame; with a mixture of atmospheric air the flame is blue; and when mixed with carbonic acid gas in different proportions, the different shades from red to purple are produced.

2d. The appearance of PHOSPHORUS has already been described to be that of a transparent substance, of a colour inclining to yellow, like a clear horn; it is specifically heavier than water,

is tough, and cuts like bees-wax, and, like it, melts in a gentle heat (99°) into a transparent fluid, and indeed with a less heat. With this heat it may be melted in water; but if the same or even a lower degree of heat (43° for instance) is applied in the open air, it melts, takes fire, and burns, producing a bright white flame with intense heat. Those who have been accidentally burnt with it complain that it gives more pain than any burn whatever. This is partly from the intenseness of the heat, and partly from the nature of the matter produced during inflammation; the phosphorus being converted into an acid.

The very small degree of heat necessary for setting it on fire, shows the very strong attraction it has for the oxygen of the atmosphere.

When the process of inflammatiou is going on, the phosphorus is gradually changing its nature. The oxygen is attracted from the atmosphere, and becomes united with it, converting it into an acid matter, then called phosphoric acid.

The following is Dr. Thomson's process of preparing phosphorus:- Let a quantity of bones be burned, or, in the chemical language, cal

A trick is played with this phosphorus, viz. setting fire to tow wrapped round a bottle, by pouring hot water into the bottle, the outside of which is first rubbed with phosphorus.