But these great and sudden variations of the heat of .a furnace are quite incon venient in most chemical processes. In the greater number of chemical opera tions, therefore, it is much more con venient to use charred fuel, than the same fuel in its natural state.
There are, at the same time, some kinds of fossil coal, which are exceptions to what has now been delivered in gene ral. We meet with some of them that leave a smaller proportion of ashes than others, and the ashes of some are not so liable to melt in violent heats. There is one species too, such as the Kilkenny coal of Ireland, and which occurs like wise in some parts of this country, that does not contain any sensible quantity of water, or other such volatile principles. But this may be called a sort of native charcoal. It has the appearance of or dinary coal, but, when thrown into the fire, does not emit smoke or soot. It merely becomes red, gives a subtile blue flame, and consumes like charcoal ; only it lasts surprisingly long, or continues to gives heat for a very long time before it is totally consumed. But it cannot be made to burn so as to produce a gentle heat. If not in considerable quantity, and violently heated, it is soon extin guished.
In using this kind of fuel, it is proper to be on our guard against the dangerous nature of the burnt air which arises from charcoal of all kinds. Charcoal burns without visible smoke. The air arising from it appears to the eye as pure hnd as clear as common air. Hence it is much used abroad by those who are studious of neatness and cleanliness in their apart ments. But this very circumstance should make us more watchful against its effects, which may prove dangerous, in the high est degree, before we are aware of it. The air arising from common crude fuel is no doubt as bad, but the smoke renders it disagreeable before it becomes danger ous. The first sensation is a slight sense of weakness : the limbs seem to re quire a little attention, to prevent fall ing. A slight giddiness, accompanied by a distinct feeling of a flush, or glow in the face and neck. Soon after, the per son becomes drowsy, would sit down, but commonly falls on the floor, insensible of all about him, and breathes strong, snoring as in an apoplexy. If the person is alarmed in time, and escapes into the open air, he is commonly seized with a violent headach, which gradually abates.
But when the effect is completed, as above described, death very soon ensues, unless relief be obtained. There is usually a foaming at the mouth, a great flush or suffusion over the face and neck, and every indication of an oppression of the brain, by this accumulation of blood. The most successful treatment is, to take off a quantity of blood immediately, and throw cold water on the head repeated ly. A strong stimulus, such as hartshorn, applied to the soles of the feet, has also a very good effect.
The fifth and last kind of fuel is wood, or fossil coals, in their crude state, which it is proper to distinguish from the char. coals of the same substances. The dif ference consists in their giving a copious and bright flame, when plenty of air is admitted to them, in consequence of which they must be considered as fuels very different from charcoal, and adapted to different purposes. See FLAME.
Flaming fuel cannot be managed like the charcoals. If little air be admitted, it gives no flame, but sooty vapour, and a diminution of heat. And if much air be admitted, to make those vapours break out into flame, the heat is too violent. These flaming fuels, however, have their particular uses, for which the others are far less proper. For it is a fact, that flame, when produced in great quantity, and made to burn violently, by mixing it with a proper quantity of fresh air, by driving it on the subject, and throwing it into whirls and eddies, which mix the air with every part of the hot vapour, gives a most intense heat. This proceeds from the vaporous nature of flame, and the perfect miscibility of it with the air.
As the immediate contact and action of air is necessary to the burning of every combustible body ; so the air, when pro perly applied, acts with far greater advan tage on flame, than on the solid and fixed inflammable bodies : for when air is ap plied to these last, it can only act on their surface, or the particles of them that are outermost : whereas, flame being a vapour or elastic fluid, the air, by proper contri vances, can be intimately mixed with it, and made to act on every part of it, exter nal and internal, at the same time. This great power of flame, which is the conse quence of this, does not appear when we try small quantities of it, and allow it to burn quietly, because the air is not inti mately mixed with it, but acts only on the outside, and the quantity of burning matter in the surface of a small flame is too small to produce much effect.
But when flame is produced in large quantity, and is properly mixed and agi tated with air, its power to heat bodies is immensely increased. It is therefore peculiarly proper for heating large quan tities of matter to a violent degree, espe cially if the contact of solid fuel with such matter is inconvenient. Flaming fuel is used for this reason in many ope rations performed on large quantities of metal, or metallic minerals, in the making of glass, and in the baking or burning of all kinds of earthen ware. The potter's kiln is a cylindrical cavity, filled from the bottom to the top with columns of wares, the only interstices are those that are left between the columns ; and the flame, when produced in sufficient quantity, proves a torrent of liquid fire, constantly flowing up through the whole of the in terstices, and heats the whole pile in an equal manner.
Flaming fuel is also proper in many works or manufactories, in which much fuel is consumed, as in breweries, distil leries, and the like. In such works, it is evidently worth while to contrive the furnaces so, that heat may be obtained from the volatile parts of the fuel, as well as from the fixed ; for when this is done, less fuel serves the purpose than would otherwise be necessary. But this is lit tle attended to, or ill understood, in many of those manufactories. It is not uncom mon to see vast clouds of black smoke and vapour coming out of their vents. This happens in consequence of their throwing too large a quantity of crude fuel into the furnace at once. The heat is not sufficient to inflame it quickly, and the consequence is a great loss of heat. See LABORATOY.