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In all the permanently elastic fluids, as the atmosphere, anil the gases, the substances of which these bodies are in part composed, are united to, or dissolved in, such proportions of caloric as completely to prevent their particles from coming within the sphere of mutual attraction. On this circumstance their permanent elasticity depends; nor has the most powerful compression to which they have ever been submitted brought their particles near enough to each to feel the attractive influence. On the contrary, the particles of those substances repel each other at indefinite distances. Thus the receiver of an air pump may be considered as full of air if it contains the least quantity, because as a part is removed, what remains instantly expands and fills its place, and this takes place until a perfect vacuum is formed.
It is on this principle that a celebrated writer has calculated, that if the pressure of the atmosphere could be removed, a cubit foot of air on the surface of the earth, would fill all the space between us, and the fixed stars.
OP THE EXPANSIVE POWER OF CALORIC.
18. In general, all bodies are expanded by an increase of temperature and contracted by its diminution. The ratio of expansion however differs greatly in different substances. Thus liquids expand by the same degrees of heat much more than solids, and aeriform bodies most of all. There is a difference also in the same class of substances. Thus by the same increase of heat, one solid, or one fluid expands more than another.
Illus. On making experiments on the expansibility of all the malleable metals, and many of their alloys, it has been ascertained that no two of them expand equally, when submitted to equal increments of heat.
Exp. 1. Fit to a bar of iron or large iron wire, a ring of metal so that the bar will just pass through it when cold. Then heat one end of the bar, or wire and it will not pass through the ring.
2. Enlarge the ring, so that the iron will pass it when red hot. Then take a bar of copper of the exact size of the iron when cold ; heat this red hot and it will be found too large to pass the ring.
Corol. The metals are expanded by heat, but some of them more than others.
19. The same increase of heat expands some liquids more than others.
Exp. Take two glass tubes terminated at one end by large bulbs; fig. 17, fix a mark about half way up the tubes, and at the same height on each ; fill one of them up to the mark, with water, and the other with alcohol; then place the bulbs of both in the same vessel of boiling hot water.— Both of the fluids expand and rise up in the tubes, but the alcohol rises about twice as high above the mark as the water.
20. The expansibility of aeriform bodies of different kinds, is in the same ratio, at equal increments of caloric.
Obs. The degree of expansion which a body of this kind undergoes, is equal to 1-483 parts of its bulk for each degree of Fahrenheit, between the freezing and the boiling points.
Exp. 1. The expansion of air may be shown by partly filling a bladder by blowing into it; then tying the mouth so that none of it can escape, and holding it near the fire. The bladder soon becomes distended, and may .Joe burst with an explosion by continuing the heat.
 
 
2. A more elegant experiment is, to take a glass tube terminated by a bulb, 4g and fill the tube about half way with water. Then immerse the end of the tube in a vessel of water, fig. 18, and apply heat to the bulb. As the heat rarities the air, the water will be seen to descend in the tube.
21. When we approach an ignited body, we become sensible that it emits caloric without touching it; and if a thermometer be carried near it, this indicates an increase of temperature.
Obs. The caloric thus imparted is called radiant caloric. It is not conducted, but passes through space independently of other matter with amazing velocity. Thus the sun radiates his heat to the earth at the rate of two hundred thousand miles in a second.
23. Radiant caloric, like light is reflected by polished surfaces, and in respect to the angles of incidence, and reflection, it follows the same laws.

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