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Molybdenum, forming molybdic acid. Obs. This catalogue contains all the substances which at the present day are considered as primary, or simple ; and they are arranged here merely as such. The first and second classes, and the first subdivision of the third class may be considered, in the present state of knowledge, as an exhibition of 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 substances 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 and potassium have been known and recognised 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 that 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 he 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.
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 bylieat, 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 beat.
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.
Obs. 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 points of view, viz. as the antagonist 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 solid becomes a fluid, as in the fusion of metals, melting of ore, &c.
Ofo. As toe 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.

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