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evolve heat. In slaking lime, or in mixing plaster of Paris, a considerable degree of latent caloric is evolved, from the water changing its fluid for a solid state. Water in becoming ice gives out its latent caloric, and that so freely as to render the progress of freezing comparatively slow.

Electricity will also produce 'caloric; but the most intense heat known has been obtained from galvanism. By means of the galvanic battery of the Royal Institution the most powerful and surprising effects have been produced. To say nothing of the metals, quartz, the sapphire, magnesia, fragments of the diamond, charcoal, plumbago, &c., seem to evaporate and disappear rapidly under its influence. The most insoluble compounds, the most compact substances with which we are acquainted, are by its means decomposed and resolved into their simplest forms.

COMBUSTION.

Simple and Compound Combustibles-Supporters of Combustion-Theory of Combustion-Result of Combustion-Combustion simply DecompositionSir Richard Phillips's Theory of Combustion-Different states of Combustion, &c.

COMBUSTION is that evolution of heat and light which accompanies certain chemical combinations and decompositions. Until late years the nature of combustion was not at all understood; and even at present it cannot be said to be perfectly known.

All bodies may be divided into two kinds, combustibles and incombustibles. Combustible bodies are again divided into simple and compound. Simple combustibles are those which cannot be decomposed; they are hydrogen (the base of water), carbon (the base of charcoal), sulphur,*

* Sulphur is the only simple combustible which nature offers pure and in abundance. It is found in the earth, and also in depositions on the surface of the earth, but most plentifully in the neighbourhood of volcanoes. It is also found in combination with most of the metals. It is extracted in the large way from pyrites; and it is said that 15,000 tons have been imported annually for the purpose of making gunpowder and sulphuric acid.

phosphorus,* boron,† and the metals; of these hydrogen and carbon are the principal. Compound combustibles are those which are formed by the union of two or more simple combustibles. A candle is a compound combustible, and so is coal, being composed of hydrogen and carbon.

There are also certain substances called supporters of combustion, which are not in themselves combustible, but whose presence is necessary for the act of combustion : the chief of these is oxygen; chlorinet and iodine§ are also supporters of combustion, but unlike oxygen, they are not supporters of life. Combustion is a play of affinities between the combustible|| body and the supporter of combustion; the combustible body has a stronger affinity for the supporter of combustion than the supporter has for caloric, which keeps it in a gaseous state; it therefore unites with the ignited body, and allows the caloric to escape. The rapidity of combustion is also found to be in proportion to the quantity of its supporter; this is evident from a common culinary fire, which increases in intensity in proportion to the supply of air it receives.

In combustion in common air the oxygen is always con

* Phosphorus abounds in the mineral, vegetable, and animal kingdoms. In the mineral kingdom it is found in combination with lead and iron, and more particularly with calcareous earths. In Spain there are whole mountains composed of lime combined with this substance. The vegetable kingdom abounds with it, but it is principally found in plants that grow in marshy places. It is also found in wheat, s seeds, potatoes, &c. In the animal kingdom almost every part of the body contains it; but it chiefly abounds in the bones, and it is this which gives them firmness. It is also found in milk; whence may be seen the wisdom and arrangement of Providence, that this and bread should form the first food of an infant, the phosphate of lime they contain being indispensable in forming and strengthening the osseous parts of the body.

† Boron is a dark-coloured powder, and the base of various salts called borates. The sub-borate of soda, commonly called borax, is a useful ingredient in the fusion of metals.

For the nature of Chlorine see the article Gaseous Bodies.

Iodine is a simple substance procured from sea-weed, and was first discovered in 1812. It is of a dark grey colour, and metallic lustre; it melts and evaporates at a low heat, and its vapour is of a beautiful violet-colour.

A combustible body will not burn if dipped in a solution of phosphate of lime, or of muriate, sulphate, or phosphate of ammonia, with borax; the alkaline substances preventing the hydrogen combining with the oxygen.

sumed; and the weight of the products of every combustion is equal to the weight of the body before combustion, together with that of the oxygen consumed; so that instead of a body being destroyed or burnt away, as it was formerly termed, it is found to increase in weight, and that in some instances to an almost inconceivable amount. One ounce of phosphorus, for instance, if properly burnt, will produce very nearly two ounces of phosphoric acid; and 100 lbs. of metallic lead, when made into red lead, which it is by being caused to imbibe oxygen through the medium of a reverberatory furnace, will weigh

112lbs.

It used to be thought that the light evolved during combustion came from the oxygen, but it has been latterly supposed to come from the combustible body, as the colour of the light depends in all cases on the nature of the body. The combustible body is, according to this theory, said to afford light and the base, oxygen gas, oxygen and caloric; the oxygen will then, in combustion, unite with the base, and generally form an oxide, and the caloric of the oxygen will unite with the light and form fire and light. Although the product of combustion is generally an oxide, it is not always so; sometimes it is an acid, as phosphorus and sulphur when burnt in oxygen gas or atmospheric air produces phosphoric and sulphurous acid. The product of potassium and sodium+ when burnt is an alkali; but alkalies are now considered metallic oxides.

* Potassium is considered a metal, and is the lightest known, being lighter than water. It does not exist naturally in a metallic state, but is formed by voltaic action on an hydrate of potash. It is of a perfectly white colour, and has the brilliancy of silver, but through its affinity for oxygen, which is greater than that of any other known body, it cannot be exposed an instant to the atmosphere without being oxidized. If a small piece be laid upon water, it will attract the oxygen of the water with such rapidity as to decompose it and ignite the hydrogen. This is a very interesting experiment, as the potassium swims about on the surface of the water, apparently burning, with a beautiful bright flame. Potassium is the basis of potash, the vegetable alkali, which latter is properly an oxide of potassium, and together with its salts is of much use in the arts, medicine, &c. Potash is formed from the ashes of burnt wood by lixiviation; it is also prepared in wine countries from the wine lees.

+ Sodium is also considered a metal, and although heavier than potash it is lighter than water. It has a great affinity for oxygen, and

The nature of combustion will be rendered further intelligible by simply considering the burning a common candle. In lighting a candle, which must be done by the application of flame, a small portion of the wick is first decomposed or burnt, the tallow becomes gradually melted, ascends the wick by capillary attraction, is decomposed, and exhibits itself in flame. The component parts of tallow are hydrogen and carbon; the hydrogen unites with a portion of the oxygen and forms water, which is carried off in vapour; and the carbon, when the combustion is perfect, unites with another portion of the oxygen and is carried off in a gaseous state. In common tallow candles the top of the wick is always black, from the flame preventing the air from oxidizing it, and forms a support, if the candle be not snuffed, for an accumulation of carbon, which is produced through the imperfect combustion. In wax candles, and also in Palmer's patent metallic-wicked candles, the wick is made so as to lean aside out of the flame; the combustion is then complete, and the wick neither collects soot nor requires to be *snuffed. The argand lamp is so constructed that the combustion is complete, and consequently it does not smoke. This is effected by the wick being of a circular shape, and so contrived that a constant supply of air is admitted to every part of it, which affords sufficient heat to burn the smoke as it is formed.

Sir Richard Phillips explains the theory of combustion in the following manner :-"Combustion," he observes, "is a case of decomposition and recomposition, and the intermediate effects or accidents are heat and light. A combustible body contains hydrogen and carbon. The air in which the combustion takes place consists of oxygen

will decompose water, yet not with that violence as to produce flame. Sodium is the base of the mineral alkali Soda, which, like potash, together with its salts, is very much used in the arts, medicine, &c, The two alkalies potash and soda, particularly the latter, are indispensable to the soap-maker, the glass-maker, and the dyer. Soda is obtained from sea-weed, and also from salt, which is a chloride of sodium; it is also found in great plenty combined with various substances. * If a common candle be inclined more than 30° from a perpendicular, it will not require to be snuffed.

+ See "A Million of Facts," by Sir R. Phillips.

and nitrogen; and if a definite bulk of air is employed the oxygen disappears, and may be found in the products of the combustion. To understand the process it is necessary to affix a precise idea to gas, which, in a word, is atoms in intense orbit motions resulting from primary force in right lines, and the reaction of other atoms in the space. Oxygen then consists of such atoms, and of course if they are fixed they transfer their momenta to bodies fixing them : if to atoms of hydrogen, they condense as water, with heat to surrounding bodies; and if to carbon, they form carbonic acid, with heat to surrounding bodies. Combustion begins by applying heat or excitement, as lighted paper, or a taper, to the combustible. This melts the tallow, and raises into gas the latent hydrogen in excitement, that is, in orbits too large for the space; and a vacuum is created, which the oxygen fills with eight times the force, becoming fixed by the hydrogen both in atoms of water, and by the carbon in carbonic acid. The oxygen is thus fixed, and the air so far decomposed, but the friction at the wick raises an intense local heat, which unable otherwise to escape, acts on the atoms of air and creates in them a general propulsion, which we call light, the heat being the concentrated motion at the spot."

Inflammation, Ignition, and Detonation or Explosion, are terms used to express the different states of combustion. Inflammation is when the combustion takes place while the combustible is in an aeriform state, as the flame of a candle. Ignition is when the combustible remains in a solid state, as charcoal. Detonation or Explosion is caused by the combustible being suddenly converted into a gaseous state, and thereby giving a violent impulse to the surrounding air.

LIGHT.

Action of Light in the Animal and Vegetable Kingdoms-Nature of LightIts Velocity -Direction-Refraction-Reflection-The Eye described-Vision explained-Colours, &c.

LIGHT consists either of small particles emanating from a luminous body, or it is an excitement produced by un

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