public in 1750. The science of Electro-Magnetism, originated in 1820; when Oersted of Copenhagen discovered the peculiar action of a galvanic current on a magnetic needle. The initial discovery in Thermo-Electricity, was made by Dr. Seebeck, in 1822. Our history of Meteorology must be so brief that it will be deferred until we come to that science in its place.

We proceed to speak briefly of Ceraunics; taking the divisions in the order above named. (p. 361).

§ 1. The science of Calorics, relates to the phenomena of heat, and the laws of its action. The name is from the Latin, calor, heat: and the name caloric, has been applied to the agent which causes the sensation of heat, and which has long been regarded as a subtile, imponderable fluid, self-repellent, capable of penetrating ponderable substances, and perhaps uniting with them in definite combinations: though recent discoveries strongly favor the extension of the undulatory theory to heat, as well as to light. The sources of heat, are the sun; combustion, and other chemical action; friction, and other mechanical action; electricity and galvanism; and animal life. Caloric is conceived to exist in two different states; sensible and latent. Sensible caloric, is that which affects the thermometer, and produces the sensation of heat; while latent caloric, though supposed to be present in bodies, does not affect the thermometer or the senses; perhaps because it is, as it were, chemically combined with the ponderable substance.

Caloric is supposed to emanate from all warm bodies, either by radiation or conduction. It is said to be radiated, when it darts forth with great velocity, like light: and it is said to be conducted, when it passes slowly from particle to particle of the surrounding bodies. Caloric radiates best from dark and rough surfaces; and is best conducted by the metals; more slowly by wood and earths; and in liquids it is carried upwards chiefly by circulation. Radiated caloric, meeting any substance, is, like light, either reflected, absorbed, or transmitted; and in the latter case refracted, as in the solar prismatic spectrum. It is best reflected from bright and polished surfaces; and absorbed by dark and rough ones. The cooling of bodies is, of course, attributed to the abstraction of caloric, from their giving out more than they receive. The effects of caloric, are the expansion of nearly all bodies, except melting ice, or antimony, and heated clay; the liquefaction of solids, and the vaporization of liquids; in which cases a certain quantity of caloric is supposed to become latent, or chemically combined; and, finally, the production of light, electricity, and chemical action.

§ 2. Electricity, so named from Expov, the Greek word for amber, treats of the phenomena first discovered in that substance, when subjected to friction; and ascribed to an extremely rare or subtile fluid, pervading the material world. If we rub smooth glass with a silken cloth, both substances become electrically excited; as is proved by their attracting pith balls, or other light bodies near them. This is explained, according to Dr. Franklin's theory, by supposing that the glass becomes charged with an excess of the electric fluid, that is positively excited; and that while this fluid repels its own

particles, it attracts those of ponderable substances, as of the pith balls. The silk is supposed to be deprived of the electric fluid, that is, to become negatively excited; and hence to attract the pith balls which contain the electric fluid, by attracting the fluid itself. The pith balls which are attracted by the glass, become charged with the fluid, on touching the glass; and are then repelled by the self-repellent power of the fluid: but two negatively excited bodies repel each other, either because ponderable matter, when deprived of the electric fluid, is self-repellent, or because they are attracted in opposite directions by the electric fluid in the surrounding air. The electric excitement is strongest when the air is cold and dry; for moist air conducts and distributes the electric fluid.

Those substances along or through which the electric fluid passes freely, are called conductors, or non-electrics; as metals, charcoal, and water but those substances which will not permit the fluid to move freely over them, are called electrics, or non-conductors; as glass, rosin, and silk; which latter becomes negative when rubbed with glass, but positive when rubbed with rosin or sealing wax; the resins being among the strongest negative electrics. When an excited electric is brought in contact with another in a neutral state, the latter becomes similarly excited by conduction: and on this principle the prime conductor of the electric machine collects the electric fluid from the glass plate or cylinder. This principle applies also to the interior of the electric jar, commonly called the Leyden jar; when its knob is brought near to the electric machine: but the exterior of the jar becomes oppositely charged, by induction; which is attributed to the electric fluid on one side, repelling that on the other. Electric excitement may also be produced by chemical action, by heat, and probably by mere contact of bodies, though in a feeble degree.

§ 3. To Galvanism, belongs the study of electrical currents, produced continuously, by chemical action; with their direct effects, and the mode of producing them. This science originated with Professor Galvani, of Bologna; who, in 1790, observed that if a piece of zine, and another of silver were brought in contact, the one with a muscle, and the other with a nerve of a frog recently killed, muscular convulsions would ensue, whenever the two metals were brought, at the same time, in contact with each other. Galvani believed this effect to be produced by a peculiar animal electricity; while Volta maintained that it was owing to common electricity developed by mere contact of the metals: but Dr. Wollaston maintained that the electric excitement was produced by the chemical action of the moist animal matter on the more oxidable metal; which last is now fully proved to be the chief cause. So rapid was the progress of this new science, that in 1800, Nicholson and Carlisle applied it to the decomposition of water; and in 1807, Davy was enabled, by its means, to discover the metallic bases of the fixed alkalies.

When a plate of zinc, and another of copper, are dipped, without touching each other, into any acid solution, the zine, by the action of the acid, is supposed to acquire an electro-positive state; which enables it to decompose water, and seize on the oxygen;

forming an oxide, and from this a salt, of zinc; in which the free electric fluid of the zinc is probably combined; while the remaining zinc attracts more of that fluid from contiguous bodies. The hydrogen of the water, being set free, probably carries with it the electric fluid of the decomposed water; and is both repelled from the zinc and attracted by the copper; to which it conveys a charge of the electric fluid. If, then, wires be attached to the two plates, and brought in contact, a galvanic current will pass along them, from the copper to the zinc plate; forming what is called a simple voltaic circle. If we have a series of cells, all containing an acid solution, the zinc plate in each cell having a metallic connection with the copper plate in the next, they will form a compound circle, or galvanic battery: in which, as in the former case, the wire from the last copper plate is called the positive, and that from the zinc end, the negative electrode; the ends of the wires being usually termed poles. If these poles be brought near to each other, provided there be at least 50 pairs of plates in action, they will produce vivid ignition, and other chemical effects: and the current may also be used to charge a Leyden jar, or to produce the other effects of common electricity.

§ 4. Magnetism, so named from the Greek, μayvŋs, a loadstone, treats of the peculiar properties first discovered in the loadstone, or native magnet; with their applications, and the means of observing them. The loadstone, is a complex oxide of iron, with a small proportion of silex and alumina. On presenting it to iron filings, it attracts them; and two opposite parts of it, which collect them most abundantly, are called the poles. If a piece of soft iron be brought near to a magnet, it becomes magnetized by induction: the part which is nearest to the north pole of the magnet becoming a south pole, and vice versa. As dissimilar poles always attract each other, and similar poles repel each other, the preceding fact explains why the magnet attracts soft iron; and why it points in a fixed direction in regard to the earth, which really acts as a large magnet would, upon the compass needle. The magnetic poles of the earth do not coincide with its geographic poles; and hence arises the variation of the compass. As we approach either magnetic pole of the earth, the dissimilar pole of the needle is attracted downwards, and hence results the magnetic dip. The property by which one magnet acts upon another, is called polarity.

Soft iron, is speedily magnetized by induction; but loses its magnetism, as soon as the magnet which excited it is removed. Hard steel, on the contrary, is magnetized slowly; but retains its magnetism for a long time. The best mode of magnetizing a bar, is by placing the opposite poles of two strong magnets at the ends of it; then rubbing it from the centre towards the ends with the poles of another magnet; each end being rubbed with a pole dissimilar to that which is to be produced; which is the method of single touch: or else rubbing it from end to end with two dissimilar poles kept near to each other; the rubbing north pole being nearest to that end which is to become a south pole; in which consists the method by double touch. Pieces of soft iron, called keepers, or armatures, are

often kept in contact with the poles of magnets, to strengthen them by inductive action.

$5. Electro-Magnetism, and Thermo-Electricity, are kindred branches of electric science, treating of the relations between galvanism, and magnetism and calorics. It was first discovered by Professor Oersted, in 1820, that a galvanic current has a peculiar action on a magnetic needle placed near it. Supposing the current to pass through the centre of a watch, from the face to the back, it tends to carry the north pole of any magnet around it, in the direction in which the hands of the watch move; and the south pole in the opposite direction. If the magnet, or needle, turn on a pivot, exterior to the galvanic current, it will soon come to rest, in a transverse position, as the above forces require. These facts were applied by Schweigger, to the invention of the galvanometer; in which, if a very feeble galvanic current circulate around a coil of insulated wire, it will be rendered manifest, by acting on a magnetic needle suspended within the coil.

In 1820, Arago discovered that a bar of steel can be magnetized by a galvanic current made to circulate around it; and hence, that a spiral coil of insulated wire, so long as a galvanic current is passing around it, acts as a magnet; manifesting polarity; attracting other magnets; and speedily magnetizing a steel bar placed within it. This furnishes a new mode of forming artificial magnets; and it led to Ampère's theory, that the peculiar properties of magnets are owing to galvanic currents circulating around their elementary particles. On this ground, it is now generally admitted that the earth's magnetism is produced by such currents, circulating around it, and caused by the heat of the sun. This is farther confirmed by Dr. Seebeck's thermo-electric discovery, that if a ring be formed of two metals, onehalf of it, for example, of bismuth, and the other of copper, soldered together, then, on heating one of the junctures, a galvanic current will pass through the same, from the bismuth to the copper.

§ 6. Meteorology, so named from the Greek Tɛwpa, meteors, relates to atmospherical phenomena; their causes, and the means of observing and foretelling them. This science, if it has now reached that dignity, engaged the attention of Aristotle, and Theophrastus; and, in more recent times, of Leslie, Dalton, and other philosophers: and it is at present the subject of profound study and careful observation. Of our own countrymen, Dr. Franklin first observed that all extensive storms, in the United States, travelled towards the northeast and Mr. Redfield first traced their course from the southern to the northeastern states; while he ascribed them, generally, to a gyratory or whirling motion of the air. Mr. Espy, who has made extensive and important researches on this subject, ascribes their formation chiefly to the action of heat: and Dr. Hare considers electricity as an equally prominent agent in their production.

If the air be heated, at any one place, it becomes rarified, and rises, as balloons do; spreading itself out above; while currents of cooler air converge to that place below, to supply the partial vacuum. This principle explains the trade winds, blowing towards the equator; and the monsoons, of the Indian ocean. (p. 163). As the heated air

rises, it becomes cooler; and the aqueous vapor which it contains, is partly or wholly condensed, thus forming a cloud, or mist; which, by the aggregation of small drops, causes rain, or, if it be sufficiently cold, hail, or snow. If the surface of the earth be cool enough to condense the aqueous vapor in the contiguous air, it causes the deposition of dew. The dew point, is that temperature at which the condensation commences: and as it varies with the quantity of vapor in the air, being the highest when there is the most vapor, it furnishes the best hygrometer, or measure of the humidity of the atmosphere. We regret the want of room to explain this subject more fully.

CHAPTER V.

CHEMISTRY.

CHEMISTRY, is that branch of Acrophysics, which treats of the composition of all ponderable substances; their sensible properties and mutual relations; and the effects produced upon them by cohesion, affinity, light, heat, and electricity. The name is derived from the Arabic or Egyptian word kimia; originally applied to Alchemy, and signifying the occult science. The general properties of ponderable substances, or those which can be collected and weighed, including solids, liquids, and gases, have already been referred to, in the introduction to this department; and the imponderables, as they are often termed, have formed the subject of the two preceding branches. Of course the study of Chemistry is aided by that of light, heat, and electricity; while it not only reflects light upon them in return, but becomes itself subsidiary to the natural and medical sciences which are to follow. The applications of Chemistry, extend throughout the wide range of the physical arts, wherever it is desirable to change the state or composition of the materials employed: and hence it ranks among the most useful of the sciences.

The Greeks ascribed the invention of Chemical science to Hermes, or Mercurius Trismegistus, called by the Egyptians, Thoyt; and, in honor of him, they gave to this branch the name of the Hermetic art. It was brought from Egypt to Greece, about 460 B. C., by Democritus; who had learned to soften ivory, and to make glass. If we except the working of the metals, little was known of this science by the ancients; and that little was obscured, rather than enlarged, by the reveries of the earlier alchemists. Alchemy, appears to have been first mentioned in the writings of Maternus, about A. D. 330; and it was fully introduced into Europe by Rhazes, and Geber, Arabian chemists of the ninth century. The object of Alchemy, was to discover an imaginary substance, called the philosopher's stone, elixir vitæ, universal solvent, or grand catholicon; which would transmute all the other metals into gold, and prevent or cure all diseases. This research, though of course a failure, led to several important discoveries; as those of gunpowder, of sulphuric, nitric, and muriatic acids, and of phosphorus, antimony, and zinc.