Is the presence of atmospheric air necessary to combustion ? Combustion cannot take place in a vacuum; no combustible body can burn without atmospheric air, or at least without oxygen, which is a component part of atmospheric air*.

In what do combustible bodies differ from each other?

Combustible bodies differ from each other principally in the rapidity with which they absorb oxygen, and in the proportion of it which they can take up, to form the new compound.

What is the effect of these properties in the act of combustion?

The greater the portion of oxygen gas which any combustible body is capable of decomposing, the greater will be the heat which is produced during combustion.

How is it known that oxygen unites with the combustible body in the act of burning?

If a combustible substance be burnt in a sufficient quantity of vital air in a close vessel, and the product preserved, the whole will be found to be increased in weight exactly in proportion to the vital air consumed; and the combustible body will then have become incombustible§.

What is the cause of a body becoming thus incombustible?

Because when a body is fully burnt it is saturated with oxygen; at least as far as combustion can saturate it; it therefore cannot combine with any more: but some bodies may be rendered com

Sweet child of stillness, midst the awful calm
Of pausing nature thou art pleas'd to dweil,
In happy silence to enjoy thy balm,

And shed through life a lustre round thy cell."

Dr. WALCOTT.

In the Asiatic Annual Register for 1802, we are told that there is a sparfow of Hindostan which has the instinct to light up its nest in the nighttime with glow-worms, which it collects for this purpose; and that it attaches them to the inside of its nest by means of a tenacious kind of clay. See Add. Notes, No. xli.

* This may be demonstrated by placing a lighted candle under a glass jar, inverted upon a plate of water. It will be seen that the candle will go out as soon as it has consumed all the oxygen contained in the included air.

Almost all the simple substances are capable of combining with 3 doses of oxygen. Thus sulphur forms oxide of sulphur, sulphurous acid, and sulphuric acid. Phosphorus forms oxide of phosphorus, phosphorous acid, and phosphoric acid. Carbon forms carbonous oxide carbonic oxide, and carbonic acid. When these simple combustibles are united, the combinations are known by names ending in uret, as sulphuret of phosphorus, phosphuret of sulphur, &c. Dr. Thomson.

Phosphorus is an eminent instance of this increase hy combustion. If an ounce of phosphorus be properly inflamed, it will produce at least one ounce and a half of solid phosphoric acid.

A series of curious experiments of this kind may be seen in Lavoisin ↑ Chymical Elements.

bustible again, by depriving them of the oxygen which they absorbed in their former combustion*.

In the decomposition of atmospheric air by combustion, what becomes of the nitrogen gas?

As the oxygen becomes fixed in the combustible body, its càloric is disengaged; part of which combines with the nitrogen, and carries it off in the form of rarefied nitrogen gas.

What chymical name is given to burnt bodies?

Such substances are said to be oxygenized, or oxidized; that is, changed into acids, or oxides.

Does the oxygen become fixed in all combustible bodies when burnt? It is a characteristic property of the combustible body to form a chymical combination with the oxygen, which is furnished by the supporters of co.nbustion. The oxygen acquires such density by this process, that it is often extremely difficult to separate it againt.

Is it possible to separate entirely the oxygen from burnt bodies?

Yes bodies may be deoxidized in various ways‡; and in some cases the oxygen may be transferred from the burnt body to a fresh combustible body, and be made the means of producing a fresh combustion§; or it may in many cases be completely sepa

**This view of combustion authorizes us to divide almost all the pro ductions of nature into two grand classes; one of combustible bodies, the other of bodies already burar 2 in the misses and action of the former we discern the causes of inflammable meteors, the perpetual alteration of the surface of the earth, volcanoes, &c., in the existence of the latter we perceive the source of the number and diversity of acids, saline compounds, oxides, and metallic salts, which vary in a thousand ways the appearance of ores, Se." Fourerov,

+ When oil is burnt in an Argand's lamp, its carbon unites with the oxygen of the atmosphere, and for us carbonic acid gas; while its hydrogen unites with mother portion of oxygen, and forms water. Every 100 ounces of oil thas burnt produce to ounces of water. In these products of combustion the oxygen is more intentele combined than it was with caloric in the gocous state. Hence we see what a beautiful series of changes and modificatio is the elements of matter are destined to undergo, and how admirably Nature has provided for the preservation of all her productions.

That the combustion at alsoy'd produced water was known in the time of Boerhave. By presenting a coll vessel to the da me of alcohol he collected water without taste or smell, and in every respect like distilled water.

Water is a product of combustion, whose base is hydrogen, the most combustible substance we are acquainted with. To restore the combustibi lity of the hydrogea, we have on v to abstract its oxygen; which may readile de done, by mixing iron or zine fangs, and sulphuric acid with the water; tie metal becomes of chaod, and be hydrogen gas is evolved as combustible

as ever.

§ hon, zinc, anti nonr, and treenic, will bern with flame when heated mis av de of mercury. Dacing the station they ttract the oxygen from and absorb it in a more sold tarp. By this process the com

tated, and shown in its primitive or gaseous state*.

What part of bodies is it which is destroyed by combustion ?

No part that we know of. We have reason to think that eve ry particle of matter is indestructible, and that the process of combustion + merely decomposes the body, and sets its several component parts at liberty to separate from each other, and to form new and varied combinations.

What is the natural inference from this interesting fact?,

The natural conclusion is, that nothing less than consummate wisdom could have devised so beautiful a system, and that nothing short of infinite power could have so modified matter as to subject it to the operation of such laws-laws that effect so many desirable purposes, and at the same time so effectually prevent the destruction of those elementary principles which are actually essential to the preservation of the worlds.

bustible property of the mercury is restored to it. Before this, it was an in combustible substance, rendered so by its union with oxygen, and the loss of its light:

This is frequently dore for the purpose of procuring oxygen gas. The oxides of manganese, or the oxides of mercury, are exposed to a proper de gree of heat, and the gas received in a proper apparatus as it is extricated. + The following concise account of the theory of combustion I copy from Berthollet, as it explains this process in a way too plain to be misunderstood; When bodies are burnt, none of their principles are destroyed; they had previously formed together one kind of compound, and they now separate from each other, at the high temperature to which they are exposed, in order to form others, with the vital air in contact with them: such of the principles as cannot unite with the vital air, that is the earth, some saline and some metallic particles, compose the cinder. The new compounds formed; are carbonic acid, or fixed air and water: the proportion of these varies accord, ing to the proportion of the carbonic particles, and of the hydrogen that had been contained in the inflammable body.' Vol. i. page 163..

"It was said of old that the Creator weighed the dust, and measured the. water when he made the world. The first quantity is here still; and though man can gather and scatter, move, mix and unmix, yet he can destroy nothing: the putrefaction of one thing is a preparation for the being, and the bloom, and the beauty of another. Something gathers up all fragments, and nothing is lost." Robinson. See Add. Note, No. xxxvii.

§ The indestructibility of matter is beautifully expressed in the following lines by Dr. Darwin →

"Hence when a monarch or a mushroom dies,
Awhile extinct the organic matter lies;
But, as a few short hours or years revolve,
Alchymic powers the changing mass dissolve;
Emerging matter from the grave returns,
Feels new desires, with new sensations burns;
With youth's first bloom a finer sense acquires,
And LOVES and PLEASURES fan the rising fires:"

CHAPTER XIII.

OF ATTRACTION, REPULSION, AND CHYMICAL AFFINITY.

WHAT is attraction* ?

Attraction is an unknown force, which causes bodies to approach each otherf.

Which are the most obvious instances of attraction?

The gravitation of bodies to the earth; that of the planets towards each other; and the attractions of electricity and magnetism.

Are you acquainted with other instances of attraction?

Yes: attraction subsists between the particles § of bodies; and

* In compiling this chapter, I have availed myself not only of Dr. Thomson's and other systems of chymistry, but also of Dr. Duncan's Introd. to the New Edinburgh Pharmacopoeia.

† Attraction has, by some philosophers, been attributed to an inherent property of matter, and by others to the influence of some foreign agent. The former is perhaps the most probable supposition.

Sir Isaac Newton demonstrated, that the planetary attraction is the same principle as gravitation. To this principle we are indebted for the periodical flux and reflux of the tides, and for other important operations of

nature.

For this the moon thro' heav'n's blue concave glides,
And into motion charms th' expanding tides;
While earth impetuous round her axle rolls,
Exalts her wat❜ry zone, and sinks the poles.

FALCONER.

§ If common flowers of sulphur and potash be mixed and thrown into water, the sulphur will separate; but if they be previously melted together, the union will then be perfect; for the molecule will be intimately combined, and the compound if treated with water will be completely soluble. The design of this experiment is to show that chymical affinity has no sensible action but on the elementary molecule of bodies.

Chap. 13.]

ATTRACTION, REPULSION, &c.

251

it is this kind of attraction which comes under the more immediate cognisance of chymists*.

How is this kind of attraction defined in chymical language?

Whenever the force of attraction operates between particles of the same species, it is called the attraction of cohesion, or the attraction of aggregation; but when between the particles of different substances, it is called the attraction of composition, or chymical affinityt.

Can you explain with more precision what is meant by attraction of aggregations?

* All the operations of chymistry are founded on the force of attraction which nature has established between the particles of bodies, and by which force all bodies cohere. The art of chymistry employs different means to destroy this attraction of cohesion, and to form fresh substances by the means of new attractions. Take silex as an instance :-In this earth the attraction of cohesion is so strong that the most powerful acids, (one excepted) have no action upon it. But if the strength of this cohesion be broken by fusing it with an alkali, it then becomes obedient to some of the other acids, and may be held in solution by them.

+ It is from the attraction of cohesion that water in drops is always spherical, and that small particles of quicksilver are constantly of a globular figure. In consequence of the same species of attraction, particles of water and other liquids ascend in capillary tubes. If a small plate of glass be laid upon a globule of mercury, the glonule, notwithstanding the pressure, continues to preserve its round figure. If the plate be gradually charged with weights one after another, the mercury becomes thinner and thinner; but as soon as the weights are removed, it recovers its globular figure again, and pushes up the glass before it.

A piece of loaf sugar broken into fine powder, or water in the state of vapour by heat, is said to have its attraction of aggregation broken; but the smallest atom of the powder is still sugar, as is the most trifling portion of the vapour to be considered as a part of the water. In order to exemplify the latter kind of attraction, a little caustic soda may be put into a glass, and muriatic acid added to it. Both these are corrosive substances; but the compound resulting from them will be found to be our common culinary salt. An instance this, of two heterogeneous bodies producing by their action on each other a distinct substance, possessing the properties of neither of the bodies which compose it. See Add. Notes, No. vii. Also several instances of chymical affinity in the chapter of Amusing Experiments.

If several salts be dissolved in the same water, when they crystalize, each particle will find its own kind by a sort of innate polarity. To prove this, dissolve half a pound of sulphate of copper and an equal weight of crystals of nitrate of potash in separate quantities of boiling water; pour them together while hot into a flat pan, and when the water has evaporated a little the salts will shoot :-the sulphate of copper altogether in blue, the nitre in white crystals, the saine as before they were dissolved. Walker.

§ There are different kinds of aggregation, viz solid, soft, liquid, and gaseous. A stone is an instance of the first, jelly of the second, water of the third, and atmospheric air of the last

If we carefully notice two small particles of mercury while gently moved along a smooth surface towards each other, a mutual attraction of one to the other will be very evident at the moment of their union into one globule. Two smali piece of cork floating in a bason of water, if not ne re to the edge than to each other, will visibly approach, and at last come into contact.