practically that they are facts and not speculations. It must however be observed, at the same time, that it is not possible to give more than a mere specimen of the means employed in arriving at this knowledge. A practical demonstration of all the facts, on which our present knowledge of combustion is founded, would be, not only an entire departure from the plan first proposed, but also, during a lecture, wholly impracticable.

The lecturer then introduced a bell glass over a lighted candle, attached to a small board, and floating on water. The light, after a short time, became extinguished, owing to its having consumed the oxygen contained in the glass,-or, to speak more correctly, owing to the oxygen having entered into combination with the carbon and hydrogen of the candle, and there being none left to enter into any further combination. The air in the bell glass which had served for the combustion of the candle, was then passed by means of a bent tube attached to the top through lime water, which it rendered turbid. This is caused by the carbonic acid, formed by the union of the carbon of the candle, with the oxygen of the air, combining with the lime held in solution by the water, and forming carbonate of lime. The insolubility of this last body in water, is the cause of its becoming turbid. few drops of muriatic acid caused the water again to become clear. For the muriatic acid decomposes the carbonate of lime, by combining with the lime, and expelling the carbonic acid, from its superior affinity for lime. By this decomposition and recomposition, muriate of lime is formed, which being soluble in water, causes the clearness of the water to be restored. The muriatic acid was added in order to show that the lime water became turbid, owing to the combination of the carbonic acid with lime. The lecturer then burnt charcoal in oxygen gas, and proved by the application of the same tests, that carbonic acid had also been formed.

A

The consequent deterioration of the atmosphere, not only by combustion, but also by respiration, for every living being consumes oxygen and gives out carbonic acid, might lead to the supposition that the oxygen of the air is continually decreasing and the carbonic acid increasing and if so that it must eventually become so vitiated, as to be unfit for the support of combustion, and the sustenance of life. This, in fact, must have been the inevitable consequence, if means had not been appointed for the restoration of its oxygen, and the withdrawal of the carbonic acid. This important operation is effected by the vegetable kingdom. The leaves of living plants are, as it were, so many laboratories in which the air undergoes purification, and is rendered fit for the performance of its important functions. It is a remarkable eircumstance in the economy of nature, that although carbonic acid gas is highly injurious to animal life, yet it is as essential to vegetable life as oxygen gas to animal. The elementary substance called carbon, the basis of carbonic acid, enters into the composition of all vegetable substances. It is the necessary food of plants. By means of their respiratory organs, they seize the carbonic acid which comes within their reach, in order to effect its decomposition. They appropriate the carbon to themselves, and throw off the oxygen, to renovate the atmosphere. Thus, what is noxious to man, is rendered beneficial to vegetables; and the oxygen which vegetables are not in want of, is separated by them in its utmost purity for the use of man. What the animal evolves and rejects as superfluous and excrementitious, the plants receive as nutritious and vital; and what the plants reject, is vital and indispensable to the animal. This was first proved experimentally by Dr. Priestley. If a living plant or shrub be placed in a glass jar, filled with water, and inverted

over water, and exposed to the action of the sun's rays, minute bubbles of air will collect on the upper surface of the leaves, and rising to the upper part of the jar, will displace the water. The gas thus evolved by the plant will be found to be pure oxygen. In like manner, a sprig of mint, corked up with a small portion of carbonic acid gas, and placed in the light, will absorb the carbon and render the air again capable of sup. porting combustion and animal life.

Water, another of the products of combustion, becomes also decomposed in the leaves of living plants. Hydrogen, the basis of water, is also like carbon, one of the constituents, as well as the necessary food, of all vegetables. During the decomposition of water, the hydrogen is retained for their nutriment, whilst the oxygen is evolved to renovate the atmosphere. The wisdom, the simplicity, and the beneficence of these arrangements are so striking, that they cannot fail, when contemplated, of exciting admiration in every reflecting mind. "Surely," says Mr. Parkes, "nothing short of consummate wisdom could have conceived any thing half so beautiful in design, or extensively and superlatively useful in effect. When we recollect the immense quantities of oxygen which must be consumed daily by combustion and respiration; and that, notwithstanding, the atmosphere always contains the same proportion of this vital principle, we can attribute the renovation to nothing but design, and perceive in it a proof that the laws of nature must be referred, not to blind chance, but to unerring intelligence combined with infinite goodness."

Shortly after the discovery of the composition of the atmosphere, and the mode of supply and regeneration of its vital part, Sir John Pringle, the president of the royal society, in presenting the discoverer, Dr. Priestley, with a gold medal, addressed him in an elegant speech, from which the following is an extract:-"From these discoveries, we are assured that no vegetable grows in vain; but that, from the oak of the forest, to the grass of the field, every individual plant is serviceable to mankind. In this the fragrant rose and deadly nightshade co-operate : nor is the herbage nor the woods, that flourish in the most remote and unpeopled regions, unprofitable to us, nor we to them, considering how constantly the winds convey to them our vitiated air, for our relief and their nourishment. And if ever these salutary gales rise to storms and hurricanes, let us still trace and revere the ways of a beneficent Being, who, not fortuitously but by design, not in wrath but in mercy, thus shakes the water and the air together, to bury in the deep those pestilential and putrid effluvia which the vegetables on the face of the earth had been insufficient to consume."

man.

The examination of these phenomena unfolds to our view, some of the most interesting subjects of contemplation which can occupy the mind of We thus see that nothing is lost. The decomposition of one thing is only a preparation for the being, the bloom, and beauty of another. Man can destroy nothing. Here it may be also remarked, that provision has been made even for the restoration of the fallen leaves of vegetables, which rot upon the ground, and, to a careless observer, would appear to be lost for ever.Berthollet has shown by experiment, that whenever the soil becomes charged with such matter, the oxygen of the atmosphere combines with it, and converts it into carbonic acid gas. The consequence of this is, that this same carbon, in process of time, is absorbed by a new race of vegetables, which it clothes with a new foliage and which is itself destined to undergo simple putrefaction and renovation to the end of time.

There are other supporters of combustion besides oxygen: it is not therefore essentially necessary to that process. The other supporters of combustion are chlorine, iodine, and bromine.

Chlorine is one of the elementary or simple substances. It is one of the component parts of common salt, and it is from this substance that it is generally obtained for experimental pur. poses. Although common salt is, as we are well aware, a solid body, yet when decompɔsed, the chlorine, one of its constituent parts, assumes the gaseous form. This gas possesses many remarkable properties, one of which is that of supporting combustion. In fact, some bodies when introduced into this gas, inflame in it spontaneously without any previous ignition. Mr. Ollivier then illustrated the properties of this substance by introducing thin leaves of copper into a bottle containing chlorine gas. The copper on coming in contact with the chlo. rine became immediately ignited, and burnt with a red light. Chloride of copper was formed in consequence. Some powdered antimony was also thrown into another jar of the same gas, which, by its instantaneous combustion, gave the appearance of a shower of fire. Chloride of antimony was therefore the result of this experiment. The spontaneous combustion of phosphorous and turpentine in this gas, was also exhibited. In the latter experiment one circumstance is worthy of remark. Carbon, one of the constituents of the turpentine, although combustible in oxygen, is incombustible in chlorine. Hydrogen, on the contrary, another constituent of turpentine, is combustible in both chlorine and oxygen. When turpentine therefore is introduced into chlorine, its hydrogen, combin. ing with that supporter, undergoes combustion, and forms a compound of hydrogen and chlorine. but the carbon of the turpentine is deposited in the form of soot on the sides of the bottle.

Having examined the compositions and decompositions which attend combustion, we shall now consider the various forms in which it proceeds. We know that it is sometimes accompanied with flame, and also that it may exist independent of flame. For instance, coal, tallow, spirits of wine, &c., inflame, but charcoal and coke burn with little or no flame. At other times the combustion is rapid and instantaneous as in gunpowder. That species of combustion which is unaccompanied with flame, is peculiar to those bodies which require a considerable quantity of heat to be volatilized or to become converted into the aeriform state, such as charcoal and coke. For flame is aeriform matter in a state of combustion. When the combustible body retains the solid form, combustion proceeds without flame. Hence the difference in the combustion of charcoal and tallow. Charcoal is nearly all pure carbon, which requires a very high temperature to become volatilized, and tallow contains, besides carbon, hydrogen, which always assumes the aeriform state. But we must not however suppose that the flame formed by the combustion of compound combustibles, such as tallow and coal, consists of the hydrogen alone of that combustible. For when carbon is united with a certain portion of hydrogen, as it is in the compound combustibles before named, it also inflames. Hence the difference in the lights emitted from the burning of coal gas or that from pure hydrogen. For coal gas consists principally of carbon and hydrogen. We may easily satisfy ourselves that coal gas contains carbon, by introducing a piece of polished metal in its flame. Part of the carbon will be deposited in the form of soot on that body. The flame of a common coal fire is coal gas undergoing combustion. When coal is exposed to a certain heat it becomes decomposed. By this decomposition gas is formed which inflames if all circumstances are favourable to its combustion. Besides the combustible gaseous matter, heat expels from the coal an aqueous vapour, loaded with several kinds of ammoniacal salts, a thick vivid fluid resembling tar, and some gases that are not of acombustible nature.

The consequence of which is, that the flame of a coal-fire is continually wavering and changing, both in shape, as well as brilliance and in colour, so that what one moment gave a beautiful bright light, in the next, perhaps, is obscured by a stream of thick smoke.

But if coals instead of being burnt in this way, are heated in a close vessel, a retort, for instance, the whole of the products may be collected and separated from one another. The ammoniacal liquor, with the tar, may be condensed and saved. The inflammable gases may be separated from the non-inflammable, and afterwards forced out to any distance by means of pipes, to serve as the flame of candles for the illumination of a room or any other place. And the coke or fixed carbonacious base of the coal remain behind in the retort.

When coal is decomposed in this way, the retort in which it is contained is heated by means of a furnace, the combustible constituents of the coals being prevented from undergoing combustion by being excluded from the air. The simple experiment of exposing the bowl of a tobacco pipe, filled with pulverized coal and covered with clay, to a red heat, will afford a simple illustration of the manner of procuring coal gas. The coal in the bowl becomes decomposed, just the same as the coal in the retorts at gas works. The gas issues from the stem, which, on the application of a candle, inflames, and continues to burn as long as the gas is supplied. The coal in the bowl is found to be converted into coke. According to this process the whole of the combustible materials may be burned. But, on the contrary, in an open fireplace, much must inevitably be lost. We often see a flame suddenly burst from the densest smoke, and as suddenly disappear; and if we apply a light to the little jets issuing from the bituminous parts of the coal, they will inflame, and burn with a bright flame. A considerable portion of combustible matter, capable of affording light and heat, constantly escapes up the chimney. Much combustible matter is also lost from the bad management of the fire, and the unscientific construction of common fire-places.

It will not, perhaps, be foreign from this subject to offer a few remarks on the economy of the combustion of this useful substance, coal. They are taken from Accum's Treatise on GasLight. With regard to the economy of this kind of fuel, or the quantity of heat produced during the combustion of any given quantity of coal, or indeed of any kind of fuel, depends much upon the management of the fire, or the manner in which the coal is burnt. When the fire burns bright, clear, and without smoke, much radiant heat will be sent off from it; but when it is smothered up, very little will be generated; most of the heat produced will then be expended in giving elasticity to a thick dense vapour, or smoke, which is seen rising from the fire; and the combustion being very incomplete, the carburetted hydrogen gas of the coal being driven up the chimney without being inflamed, the fuel is wasted to little purpose.

Nothing can be more perfectly devoid of common sense, and wasteful and slovenly at the same time, than the manner in which chimney fires, where coals are burnt, are commonly managed by servants. They throw on a load of small coals at once, through which the flame is hours in making its way; and frequently it is not without much care and trouble that the fire is prevented from going quite out. During this time no heat is communicated to the room; and, what is still worse, the throat of the chimney being occupied merely by a heavy dense vapour, not possessed of any heating power, and, consequently, not having much elasticity, the warm air of the room finds less difficulty in forcing its way up the chimney and escaping, than when the fire burns bright, and the coal is

ignited. And it happens not unfrequently, espe cially in chimnies and fire-places ill constructed, that this current of warm air from the room which presses into the chimney, crossing upon the current of heavy smoke and aqueous vapour which escapes slowly from the fire, obstructs it in its ascent, and beats it back into the room. Hence it is that chimnies so often smoke when too large a quantity of fresh coals is put upon the fire. So many coals should never be put on the fire at once, as to prevent the free passage of the flame between them, or to prevent them becoming quickly heated, so as to give out the carburetted hydrogen gas which they are capable of furnishing, and to cause it to be inflamed. In short, a fire should never be smothered and when attention is paid to the quantity of coals put on, there is little use for the poker; and this circumstance will contribute much to cleanliness, and the preservation of furniture.

The author of a paper in the Plain Dealer asserts, that, of the various perversions of abilities, there is none that makes a human being more ridiculous than that of attempting to stir a fire without judgment, to prevent which he lays down the following rules:-1. Stirring of a fire is of use, because it makes a hollow where, the air being rarefied by the adjacent heat, the surrounding air rushes into this hollow, and gives life and support to the fire, and carries the fame with it. 2. Never stir a fire when fresh coals are laid on, particularly when they are very small, because they immediately fall into the hollow place, and therefore ruin the fire. 3. Always keep the bottom bars clear. 4. Never begin to stir the fire at the top, unless when the bottom is quite clear, and the top only wants breaking.

Another important subject in the economy of heat, is, the size of the coal. It is not generally apprehended how very wasteful the use of small coals is in the ordinary open fire-grates. Necessity makes us use the poker, particularly when the coals are small; and habit prevails even when they are large. By the constant stirring of the fire, almost the whole of the small coal passes through the bars; and, consequently, a great deal goes to the dust-hole without being burnt at all. To prove this, we need only take a shovel full of ashes and put them into a pail, and then pouring water over them, which being gently run off, will carry nearly all the light and burnt parts, and leave an astonishing quantity of bright unburnt coal, which has escaped from the fire-place, in consequence of being small.

When the grate of the fire-place is large, and the small coals are thrown behind; or when we can have patience enough to bear the cold for an hour or two, or contrive to have the fire lighted a long time before we want it, the small coal may be of some use; but the fire made with it is never strong, nor so bright, and does not burn so long as a fire made with large or round coals: it often requires the help of the poker, and produces a great quantity of breeze.

The loss in the use of small coals is more considerable to the poor, who cannot keep large fires. When they want their breakfast or dinner, the time they can spare is limited; and to have their water sooner boiling, or their meals quicker ready, they must make use of the poker, and lose a great deal of coal. This fact is so evident, that any body who wishes to make the experiment before recommended, will find that much more bright coal goes to the dust-hole of the poor man than to the dust-hole of a rich family, where, the fire-place being large, the small coal has more chance of burning.

The manner in which a candle undergoes combustion is also an interesting subject of investigation. In a candle, three things present themselves to our notice: the tallow, the wick, and the flame. The tallow is, as it were, the magazine containing the materials necessary

for the production of flame. The wick is the medium of communication between the flame and tallow. And the flame consists of gaseous matter, arising from the decomposition of the tallow, in the act of combustion. This process proceeds only at that part which is immediately in contact with the atmosphere. In the interior of the flame no combustion can take place, as the air can have no access to it. Flame, therefore, is a mere film or bubble, and not solid. It contains, however, gaseous matter, ready to take the place of that portion which is undergoing combustion.

The mode by which a constant supply of gas is insured, is worthy of remark. The tallow in the vicinity of the flame, liquified by the heat, is drawn up in the wick by capillary attraction. Here, by the intense heat to which it is exposed, it suffers decomposition, assumes the gaseous form, and undergoes combustion. When oil is burnt in lamps, precisely the same thing occurs, it rises in the wick by capillary attraction to the flame, and is there decomposed, and gas is consequently formed. We see, therefore, that gas and candle light are the same, for the tallow is actually converted into gas before it is consumed. Hence we must admire the simple yet wonderful contrivance of a common candle. It may be aptly compared to a self-generating gas apparatus. The flame performs the office of a furnace, and the wick serves to convey the combustible matter at the fire-place, or place of combustion, and also the purpose of a retort. Another circumstance is worthy of remark. We perceive that the wick, although combustible, does not undergo combustion, while surrounded with flame, because the flame protects it from the action of the atmosphere. For when the wick, by the continual wasting of the tallow, becomes too long to support itself in a perpendicular situation, the top of it projects out of the cone formed by the flame, and thus being exposed to the action of the atmosphere, is ignited, loses its blackness and is converted into ashes.

In a common candle or lamp, a portion of combustible matter escapes unburnt, which causes the formation of smoke. Smoke indicates imperfect combustion. It may arise from the want of a temperature sufficiently elevated to effect the combustion of the whole of the volatilized combustible matter. For we find that the introduction of any cold body in the flame of a candle or lamp, will cause an immediate evolution of smoke, and deposition of soot on the cold body. The smoke thus evolved, is part of the carbon of the candle minutely divided, some of which may be perceived deposed on the cold body, in the form of a black impalpable powder. This arises from the temperature of the flame being so far lowered as to prevent the ignition of the carbon evolved by the decomposition of the tallow. For carbon requires a higher temperature for its ignition, than hydrogen. Smoke may also be formed by the supply of combustible matter being greater than the consumption,-when a greater portion is volatilized than can come in contact with the oxygen of the surrounding atmosphere. Hence it follows, that with a large wick and a large flame, the waste of combustible matter is proportionably much greater than with a small wick, and a small flame. In fact, when the wick is not greater than a single thread or cotton, the flame, though very small, is peculiarly bright and free from smoke; whereas in lamps, with very large wicks, such as those of lamp lighters, there is an immeuse volume of smoke, which in a great measure eclipses the light of the flame. In order to remedy this defect of common lamps, the argand lamp has been constructed, in which an internal current of air is established, which readers the combustion perfect by the application of air on both sides of a thin flame.