sents, will be found to bear most intimately upon much of our previous inquiry, regarding the habitudes of solids, liquids, and gases, with cold and heat.

The materials generally laid in a fire-grate are paper, wood, and coal; and there is much philosophy in this seemingly simple arrangement; for these substances are placed in the order of their combustibility, they are all closely allied in chemical composition, they differ materially in physical form; paper being fragile, wood tough, and coal brittle.

A lighted match applied to the paper, instantly causes its inflammation; this kindles the wood, which does not burn so rapidly on account of its greater aggregation, and therefore allows sufficient contact of its flame to excite the combustion of the more closely aggregated coal; this soon ensues, the fire burns well, the smoke and vapor swiftly ascending through the chimney.

At first sight there appears no reason why a fire might not be kindled as readily, at the top of the grate as at the bottom, but daily experience shows us this cannot be done, and for a very simple reason, which may be illustrated as follows.

Fold a long strip of paper into the form of an ordinary "allumette," kindle one end of this, hold it horizontally, and the flame will immediately ascend powerfully from its upper side; and however it may be held, the flame will always ascend, and most rapidly when the paper is held with its kindled part pointing downwards; do these simple matters appertain to chemical philosophy? they do, and for the following reason.

The flame cannot burn without air, and as it chemically withdraws Oxygen from such portions of this wondrous medium as are in immediate contact with it,

this action mechanically disturbs the natural equilibrium of unchanged or fresh portions, at some distance around and below; these, accordingly, move swiftly onward and upward, to supply the deficiency occasioned by the flame; and it consisting of volatile matter, is urged by the force of such moving air, into a pointed or pyramidal form.

The dangerous and fatal accidents that have happened, more particularly to females, by their thin dresses catching fire, afford sad illustrations of the invariable ascent of flame.

In the case of dresses catching fire, persons generally are greatly alarmed and lose their presence of mind, and therefore run wildly to and fro, thus fanning the flames into their utmost intensity; the prudent plan is, to lie down upon the floor, and roll the hearth rug around the body to exclude the air, and thus extinguish the flames; or, if there be any bystanders at the time of the accident, the cloak of one or the coat of another, thrown around the body of the sufferer, will instantly quench the flames.

A simple experiment may serve to impress this useful fact upon the mind; spread a sheet of paper, flat upon a board, kindle the four corners of the paper, and observe how slow, how imperfect will be the combustion; and to what a small extent the flame will rise; because the air has only access to the upper part of the paper, and cannot enter below to urge the flame to its full pointed intensity of height; as it will do the instant that the sheet of paper is lifted upright, by taking hold of one corner with the tongs.

But in either case, the flame may be perfectly and

safely quenched, by spreading a thick cloth over it to prevent the access of air; it should be particularly borne in mind, that no common flame can burn without access of air; but now, to continue our investigation regarding the philosophy of applying such flame to the lower strata of fuel in the "fire grate."

It is a well-established fact, that, all substances,-not even excepting water, after a certain degree is passed, -under the influence of heat increase their mass, bulk, or volume, and diminish in their specific gravity; they, accordingly, in the case of mobile liquids, gases, and vapors, have an immediate tendency to ascend through colder and heavier portions of their own nature, or in many cases of other liquids, gases, and vapors, of totally different nature; thus, if the hand be held some inches above a flame, great heat will be felt, for the products of combustion, namely, Watery vapor and Carbonic acid, are both intensely hot, both specifically lighter, and therefore ascending through the colder, heavier air; if the hand be held under the flame, little more than actual heat of radiation will be felt upon its palm, whilst its back will feel cold, by the fresh air ascending to supply the place of the escaping products of combustion.

Let the thermometer with its blackened bulb, employed in the experiment at page 152, be held some distance above, and then the same distance beneath a horizontal flame, and in the former position it will indicate a much greater temperature than in the latter.

The reason, then, of kindling the fuel in a grate at the lower bars, is that the flame may assume its pointed form, and ascend, with its own heated products; which, gradually drying and heating the wood, and it in turn the coals, induces their combustion; then all the products

ascend through the chimney, and emerging from it into the air, if it be calm will continue ascending for a considerable height as one unbroken column, whilst fresh air enters the room, to supply the fire, still urging the ascent of its flame and heated products.

If, on the other hand, the fuel were laid in an inverted order, the paper being uppermost, this upon being kindled would immediately obey the impulse of the air, and ascend in flame with such rapidity, that the wood, unless extremely thin, would be prevented from gaining sufficient heat to burn; and then in turn, it would pass away with rapidity, without kindling the solid coals.

In all ordinary cases of the employment of coals as fuel, a portion of carbon passes away unburnt, in the ascending draught of actual products of combustion; and this carbon constitutes the chief part of the black smoke, yet being highly heated, it generally floats for a considerable time in the air.

Remark the dense volumes of smoke that issue from the lofty stacks of steam-engines, breweries, and gas works; how far they travel, through the calm air, and do not appear to blend with it; the watery vapor and carbonic acid that they contain, do really blend and dissolve in the air, but the unburnt carbon, cannot do so, it therefore remains mechanically diffused for a considerable time, until it becomes so cooled as to fall in the form of "blacks."

On the other hand, remark the dense white cloud of steam that issues from a railway engine, with what force it ascends a considerable distance in the air, and then forms small clouds; but these immediately vanish and do not travel like the carbon of smoke, because they consist of nearly pure watery vapor, and chemically dis

solve in the air without altering its transparency; the same as a lump of white sugar will dissolve in water, without altering its pellucid appearance.

Smoke, therefore, presents an example of the mechanical diffusion of a substance; and steam, an example of the chemical solution of a substance in the atmosphere; again the dust of Summer, and the snow flakes of Winter, floating in air, present examples of "diffusion;" and when they fall, of "precipitation" from such medium; but the solid dust can never return into the air save by its mechanical impulse; whilst the snow flakes, by first changing from their solid state to that of vapor, by the warmth of the air, chemically return into its vast and capacious volume.

Returning to the chemical phenomena presented by an ordinary fire, we find it a point of the utmost importance, that the products of combustion and the smoke should have free escape, and that it is absolutely necessary to admit as much air into the room as the flame consumes; if this be prevented, by nailing leather around the doors, or placing sand bags at their thresholds, or upon the window frames, the smoke, instead of ascending, will be forced into the room by a current of air descending from the chimney-pot, to supply the deficiency of that in the room.

The admission of cold or fresh air to the lower part of the fire, without occasioning the inconvenience of a draught, is one of the most difficult problems for practical solution that can be presented, and probably the simplest plan is to carry an open tube from the hearth into the external air; at all events, the fresh air should gain admission at or near the floor, and not at or near the ceiling of a room; the latter situation is the most suited