facts. The second subdivision of the third class is chiefly composed of articles of doubtful, or even of conjectural characters, their claims to be ranked as metals, being chiefly from their analogy to substance of known metallic bases. Among the number, lime, barytes, and strontites only, have exhibited direct proofs of their metallic bases. The others, as being earths, also, and as having most of them exhibited some signs of a metallic nature, have on these accounts been arranged by late writers on chemistry with the metals. Of the three articles under the third subdivision, of the third class, two of them sodium potassium have been known and recognized as metallic bodies for a series of years. Lithium is of more recent discovery, and as a metal has not been exhibited in a state of purity.

IMPONDERABLE AGENTS.

10. The imponderable agents are heat, light, electricity and magnetism. They are so called to distinguish them from substances which have appreciable weight.

OF HEAT OR CALORIC.

11. Heat is the sensation which one feels on touching a body hotter than the hand. This sensation is caused by the passage of caloric from the hotter body to the colder. Thus caloric, is the cause of the sensation called heat, and heat is the effect of the passage of caloric, or the matter of heat, into the hand,

Obs. One of the most obvious and universal properties of heat, is its disposition to seek an equilibrium, or to pass from the hotter body to that which is colder.

Illus. 1. If two bodies of different temperatures be placed in contact, or near each other, the one whose temperature is the highest constantly throws off heat from its surface, which is absorbed by the other, until the colder body has gained, and the hotter body has lost, a quantity of caloric, which bring them both to the same temperatures.

2. If two thermometers graduated exactly alike be placed at a distance from each other in the

open air, they will both indicate the same degrees of heat.

Corollary. Caloric has the power of pervading all substances, and of equalising their temperatures.

12. Caloric may exist in two different states, viz. in a state of freedom, and in a state of combination. In the former state it is capable of exciting the sensation of heat, and of enlarging the dimensions of bodies. In the latter state it neither excites sensation, nor produces obvious effects on substances.

13. Whenever we experience the sensation of heat, on touching a hot body with the hand, that body contains caloric in a state of freedom, otherwise it could not pass into the hand.

Obs. What in common language is meant by heat, is always free heat, since its other modification neither excites sensation nor affects the temperature of bodies. Thus a red hot ball of

iron contains a large quantity of free heat.

14. Combined caloric, also called latent heat, neither affects sensation, the magnitude of bodies, nor the thermometer. A knowledge of its existence therefore is acquired by experiments only. Ols. Combined caloric becomes free under circumstances to be mentioned hereafter.

OF FREE OR UNCOMBINED CALORIC.

15. Caloric, as a chemical agent, may be considered in two relations, viz. as the opposing power to cohesion, and as causing or increasing the elasticity of bodies.

16. As the antagonist to the cohesive attraction, or that power which keeps matter in masses, the effects of caloric, are to cause the particles of solids to repel each other, so as to overcome their natural attractions. When this takes place the particles easily slide over each other, and the sol

id becomes a fluid, as in the fusion of metals, mel

ting of ore, &c.

Obs. As the caloric passes out of a fused substance as a metal, the particles gradually approach, so as to regain the sphere of mutual attraction. Thus from a fluid, the metal, as it cools becomes a semifluid, or soft solid, and when the repulsive power of the heat is so weakened, by its loss, that it is completely overcome by the attraction of the particles, then the metal becomes solid as before. The forging of iron depends on this principle; its particles being so far separated by the heat, as to move among themselves by the blow of the hammer.

17. As causing, or increasing the elasticity of bodies, caloric combines with the particles of some substances, so as not only to destroy their cohesion, but so as to throw them completely beyond the sphere of each other's attraction.

Illus. In all the permanently elastic fluids, as the atmosphere, and 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 other 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.

OF 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 substan

ces.

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.

Then

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. 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.

17

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.

080

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 be burst with an explosion by continuing the heat.

18

2. A more elegant experiment is, to take a glass tube terminated by a bulb, 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 rarifies 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.

22. Radiant caloric, like light, is reflected by polished surfaces, and in respect to the angles of incidence, and reflection, it follows the same laws.

19

Exp. Provide two circular concave tin mirrors, fig. 19, of about 10 or 12 inches in diameter, and two inches deep. They are made by planishing the tin with a round faced polished hammer, and afterwards made as bright as possible by rubbing with buckskin and whiting. They may be supported in the perpendicular position by having slips of tin soldered on their backs, and then fastened to wooden stands.

Place the mirrors, being both of the same height, on a table opposite to each other, and about 10 or 12 feet apart. Place a