of coal in a parlour grate: and it is attended with the production of Heat, as is evidenced by the intense temperature produced by blowing air through melted iron, as in the Bessemer process, after the silicium and carbon have been already burnt out of the metal. But the oxide of iron, thus produced, may be decomposed and the metallic iron thus revivified, if it be subjected to the still greater temperature of a blast furnace, wherein all the natural oxides of iron are reduced by Heat alone.

The explanation of combustion, herein offered, is applicable to every case of energetic chemical union, excepting only in those where the natural chemical affinity is so strong that the instigation of an igniting temperature is unnecessary. When it is admitted that, however Heat may repel the particles of uniformly heated bodies, each particle, in acquiring this force of repulsion for itself, must, at the instant of acquisition, have been forcibly attracted to the source of Heat, the whole rationale of combustion is explained, and an illustration becomes almost needless. To state an actual case, however, we may instance the combustion of a fragment of charcoal. The mass consists of innumerable atoms of carbon, held together by an inferior force of cohesion. Each atom has a certain amount of repulsive force for all other atoms, this amount depending upon its temperature. The atoms of oxygen, in the air surrounding the charcoal, have, also, a like repulsive force corresponding to their temperature. The repulsive force, or Heat (commonly understood, in this form, as "specific Heat"), is greater than the natural chemical affinity of the oxygen and carbon, and hence they remain separate; although, in time, even charcoal becomes oxidised by silent combustion. But if a few atoms of the charcoal be ignited, or simply raised to incandescence, which, in

F

itself, is not necessarily a result of combustion, then the surrounding atoms of oxygen are momentarily attracted to the source of Heat. On the instant when they attained an equilibrium of temperature they would be ready to fly away in virtue of the very repulsive power of the Heat just acquired, but in the mean time they have been entrapped by their chemical affinity for the carbon atoms, and the compound carbonic acid detaches itself from the solid mass by reason of its acquired Heat, and remains a compound atom until it be decomposed by the intense agency of vegetable assimilation, hardly inferior in analytical power to electricity itself. The union of the pair of atoms is accomplished by great force, this being measured by the Heat evolved, and which Heat is communicated to neighbouring pairs of expectant atoms, until all have been "burned," and the supply of combustible is exhausted.

It may be added that atoms of oxygen may receive a certain amount of Heat without immediately entering into combustion with the ignited combustible atom. A certain time, however inconceivably brief it may be, is necessary for union, and if, while this is about to commence, the chosen atom of oxygen is thrust aside to make room for a fresh candidate, of the same family, the incandescence of the combustible may be dissipated in minute appropriations to a series of atoms and no combustion will then take place. Thus Count Rumford, by firing small globes of solid gunpowder through paper screens, ascertained that, in many cases, a portion of gunpowder which had been actually ignited, and a part of the surface of which had been already burnt away, was extinguished by the rapidity of its flight through the air, and the consequent communication of its Heat to the atmospheric atoms in its path, before this Heat

could be appropriated by the nitre of its own substance. The extinction of the flame of a candle by a puff of the breath is explained in the same manner.

VAPORISATION.

The conception of Heat as the counter-force of every kind of attraction, leaves no obstacle in the way of a clear and simple explanation of vaporisation and condensation, Solidity, liquidity, and gaseity are thus comprehended, respectively, as dependent upon the relations of opposite forces pervading the atoms of a given substance. If the attractive forces, of whatever kind, greatly exceed that of Heat, the atoms cohere as a solid. If Heat, considered as repulsive force, be then applied, it will, in proportion to its quantity, counteract the force of cohesion; and, if the quantity be sufficient, the force of Heat will change the solid into a liquid. In the burst, from solidity to liquidity, a large amount of repulsive force must be expended, in order to overcome the close mutual grasp of the atoms, and the force so expended is known as the "latent Heat of liquefaction." As a liquid, the atoms, still attracting each other, may receive a still further charge of Heat, whereby the liquid mass is dilated, but during this dilatation, as a liquid, the atoms never lose a certain amount of attractive force for each other. This force is sufficient, not only to resist the repulsion of considerable increments of Heat, but in the final burst from the state of a liquid into that of a vapour, a very large amount of repulsive force (Heat) must be brought into action. The heat expended in finally overcoming the attraction of the atoms of a liquid for each other is not, while thus engaged, conferable by these atoms upon others of which the attraction has not been assailed. Two atoms of a liquid, about to

separate, under the force of Heat, to a distance corresponding to the vaporous state, appropriate a large amount of Heat in the separation, and so effectually that this Heat cannot, while the vaporous state is maintained, forsake the atoms of the liquid for those of the mercury in the bulb of the thermometer. For the Heat, or repulsive force, cannot do double duty at the same time, and in the struggle between liquidity and gaseity it can only escape into the bulb of a thermometer, or into a volume of colder liquid, by abandoning the work in hand-to wit, the forcible separation of two particles which would otherwise instantly coalesce. The Heat thus exerted in restraining the ultimate attraction of the atoms of the liquid for each other is generally known as the "latent Heat of evaporation." It does not affect the thermometer, but, as has already been seen, there is, in all matter, a distinction of prime consequence between the Heat absolutely present, and that, merely, which, without permanent change of form, may be imparted by one body to another. It is only the latter form of Heat which can be determined by the thermometer, the total Heat in any substance being beyond any and every means of determination.

Vaporisation may now be examined with reference to a given quantity of water at the ordinary temperature of the atmosphere. Placing the water in an open vessel, over the flame of a lamp, the lowest stratum of the atoms of the liquid is first heated by conduction through the substance of the bottom of the vessel. To what extent this heating proceeds before the lowest atoms rise from contact with the bottom is not known. It cannot yet be positively said whether they rise with the first minute reception of Heat, or whether their inertia and that of the superincumbent atoms which they must

displace in rising, restrains them until they are completely vaporised. The result, in either case, will depend upon the temperature of the bottom of the vessel, or, more precisely, the temperature of that surface of the bottom which is in contact with the liquid. If, under a rapid application of Heat, this temperature be found to exceed the boiling point of the liquid while the mass of the latter is still cold (and as yet, perhaps, no proof has been adduced that it does not), then the conversion of the lowest atoms at once into vapour would be almost certain. A coating of fusible metal, melting at the boiling point of the liquid to be vaporised, might decide this point. The well-known fact that water may be boiled in a card-board box would derive new interest from the plating of the inner surface of the box with fusible metal, melting at 212 deg. If we fill a tin vessel with water and place it over a lamp, and if we then thrust down the tip of the fore-finger so as to nearly, but not quite, touch the inner surface of the vessel, exactly over the spot where the flame impinges, a sharp burn will be the result, although the mass of the water in the vessel may be scarcely above the freezing point. There is, also, to be considered the positive attraction which all matter is herein assumed to possess for Heat, and which attraction would, for an instant, hold down the particles of water in contact with the heated bottom of the vessel. At the instant of an equilibrium of temperature, the atoms thus held down would bound upward, and by their buoyancy rise in the water, and in the moment occupied in the arrival of other atoms of water, Heat would be accumulating in the bottom of the vessel. This attraction of matter for Heat is illustrated in the formation of minute bubbles at the bottom of a vessel of water in the course of heat