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very strong tendency to combination, we are not able to procure it in that state.

Combined caloric is that which is fixed in bodies by affinity or elective attraction, so as to form part of their constitution. By the expression specific caloric of bodies, we understand the respective quantities of caloric requisite for raising bodies of the same weight to an equal degree of temperature. This proportional quantity of caloric is thought to depend upon the distance between the constituent particles of bodies, and their greater or less degrees of cohesion, and this distance, or rather the space or void resulting from it, is called the capacity of bodies for heat.

Heat has the property of expanding bodies, or increasing their bulk. This may be observed by fitting a piece of iron to an iron ring so as just to fill it: then if the iron be heated in the fire, it will be found that it has become too large to pass through the ring; but when cooled, it contracts to the same size as before. Some metals will expand more than others.

It is supposed that the caloric forces itself be. tween the particles of bodies so as to separate them. In acting thus, it is in direct opposition to the attraction of cohesion, which keeps them together.

Fluids also expand by heat. Put water into a very small glass tube with a bulb, and apply heat to the bulb; the water will be seen to expand and fill more of the tube: as it cools, it will contract again.

Gases also increase in volume, by increase of temperature. Tie the neck of a bladder tight; when it is almost empty, lay it before the fire ; the included air will expand, and the bladder will swell

and appear full, but will return to its former state by withdrawing it from the fire. From this property of matter in expanding by heat, the thermometer becomes a measure of the heat in bodies.

Every body, therefore, whether solid, liquid, or gaseous, is augmented in all its dimensions, by an increase of sensible heat: and on the contrary, all bodies contract by an abstraction of caloric. We are still very far from being able to produce the degree of absolute cold, or total deprivation of heat; hence, we are incapable of causing the ultimate particles of bodies to touch each other.

It may be supposed, since the particles of bodies are thus constantly impelled by heat to separate from each other, that they would have no connexion between themselves; and that, of consequence, there could be no solid body, unless the particles were held together by some power which tended to unite them : this power is the attraction of cohesion.. Thus, the particles of all bodies may be considered as subject to the action of two opposite powers, repulsion and attraction, between which they remain in equilibrio. So long as the attractive force remains strongest, the body must continue in a state of solidity; but if, on the contrary, heat has so far removed these particles from each other, as to place them beyond the sphere of attraction, they lose the cohesion they had before with each other, and the body ceases to be solid.

Water gives us a regular and constant example of these facts. Whilst below 32° it remains solid, and is called ice. Above that degree of temperature, its particles being no longer held together by reciprocal attraction, it becomes liquid; and when we raise its temperature above 212°, its particles giving way to repulsion caused by the heat, assume

the state of vapour or gas, and the water is changed into an aeriform fluid.

The same may be affirmed of all bodies in nature. They are either solid, or liquid, or in the state of elastic aeriform vapour, according to the proportion which takes place between the attractive force inherent in their particles, and the repulsive power of heat acting on them; or, what amounts to the same thing, in proportion to the degrees of heat to which they are exposed. But were there no other cause affecting the solidity of bodies except the powers of attraction and repulsion, they would become liquid at an indivisible degree of the thermometer, and would almost instantaneously pass from the solid state of aggregation to that of aeriform elasticity. Thus water, for instance, at the very instant when it ceases to be ice, would begin to boil, and would be transformed into an aeriform fluid, having its particles scattered indefinitely through the surrounding space. That this does not happen, must depend, upon the action of some third

power.

The

pressure of the atmosphere prevents this separation, and causes the water to remain in the liquid state until raised to the temperature indicated by 212° ; the quantities of caloric, which it receives in the lower temperatures, being insufficient to overcome the

pressure of the atmosphere. Whence it appears, that, without this atmospheric pressure, we should not have any permanent liquid, and should only see bodies in that state in the very instant of melting; for the smallest addition of caloric would then instantly separate the particles, and dissipate them through the surround ing medium. Besides, without this atmospheric pressure, we should not even have any proper aeri

form Auids; because the force of attraction would be overcome by the repulsive power of caloric; and the particles of bodies would separate themselves indefinitely, having nothing to give limits to their expansion, unless their own gravity might collect them together so as to form an atmosphere.

It may be admitted, therefore, as a general principle, that almost every body in nature is susceptible of three several states of existence, solid, liquid, and aeriform ; and that these states depend upon the quantity of caloric combined with the body.

The elastic aeriform fluids are expressed by the generic name of gas ; and in each species of gas, a distinction is made between the caloric, which, in some measure, serves the purpose of a solvent, and the substance which, in combination with the caloric, forms the base of the gas. Thus water, united to a sufficient quantity of caloric, is called aqueous gas; ammoniac saturated with caloric, is called ammoniacal gas, &c.

Caloric, when free, appears to move in the form of rays, and to be capable of being reflected in the same manner as light. The calorific part of the solar rays, or those which occasion heat, are condensed by a lens, ora mirror, as well as those which produce light; and the rays of heat from any burning body, or even of a body heated, although not in a state of combustion, are thrown off in a radiating manner. If two polished metallic mirrors be placed opposite to each other, at several feet distance, and if a pan of burning coals, or a heated piece of iron, be held in the focus of one of them, a thermometer placed in the focus of the opposite one, will be immediately affected as if it had been

held close to the heated matter; and this will be the case, even if the heated body is not luminous or incandescent, as hot water, for instance; so that the invisible rays of heat also are reflected like those of light. The chief part of the heat received from a common fire is in the form of radiant heat; and whatever kind of construction will most promote the reflection of radiant heat into the room, will be the most advantageous form of the chimney. It is upon this principle that the grates introduced into common use by Count Rumford are so much

pre. ferable to all others.

The effect of the solar rays upon bodies differs much according to their colour; black and dark coloured bodies are more heated than white ones; the latter throwing off the rays, while the former absorb them. For this reason, black clothes are more heated by the sun than white ones. Polished surfaces, also, which reflect best, do not absorb so much heat as rough surfaces.

The boiling or ebullition of liquids is a phenome. non which depends upon the liquid being converted into vapour by a certain degree of temperature; consequently, those liquids which assume the vą. porous or aeriform state at the lowest temperature are most easily made to boil. The ebullition, or the noise and motion of the liquid in boiling, is occasioned by small quantities of vapour being formed at the bottom of the vessel, which rise by their lightness in a globular form, and break at the surface. The ebullition of liquids is easier in proportion as the pressure to which they are subjected is less ; thus water, which boils only at 212° Fahr. in the air, will boil with a much less degree of heat in an exhausted receiver of the air pump; and it

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