duced, as far as convenient, into one of the nostrils, and then be brought into contact with a piece of zinc placed on the tongue, a sensation not unlike a strong flash of light, will be produced in the corresponding eye, at the instant of contact.

3. Professor Volta, in the year 1800, constructed a pile, for condeusing, retaining, and communicating a perpetual current of the galvanic influence. This pile consisted of alternate layers of plates of zinc, silver, and moistened paper. On forming a communication between the top and the bottom of this pile, a smart shock is felt, which will decompose water. Sparks may be obtained,-gunpowder and spirits of wine ignited, and metallic wires rendered red hot, and even fused. The effect is increased by enlarging the size of the plates, and by moistening the paper with diluted muriatic acid instead of water. Conductors,

in galvanism, are either perfect or imperfect. The first consists of metallic substances and charcoal;—the last, are water, and the oxydating fluids, as the acids, and indeed all substances containing these fluids. Every galvanic combination must consist of three different conductors, not entirely of one class, and the conductors of one class must have some chemical action upon those of another. These combinations are termed galvanic batteries.

4. The battery consists of an oblong vessel of baked wood, about three inches deep and as much broad. In the sides of this vessel, grooves are made oppo-ite to each other, and about one-eighth of an inch in depth. In each pair of grooves a double metallic plate, viz. a plate of zinc and a plate of silver soldered together at their edges, are cemented. By this means the wooden vessel is divided into several partitions. Those cells are afterwards filled almost to the top with water, or any other fluid, and thus the whole will form a battery, consisting of various repetitions of silver, zinc, and fluid. Several of these batteries may be united by iron cramps, which act as one battery. Substances, till lately considered as simple bodies; as, the earths, sulphur, and other bodies, have been decomposed by the galvanic influence. Sir Humphry Davy, on bringing potash and soda within the action of a powerful voltaic battery, found that metalline substances were produced, to which he has given the name of potassium and

sodium. He has also, by analogous means, discovered that not only the fixed alkalis, but several of the earths, are metals combined with oxygen; and hence have been found the new substances strontium, calcium, magnesium, glycinium, &c.-(See our next chapter.)

Select Books on Natural Philosophy.

Gregory's Economy of Nature, 3 vols, svo. Ferguson's Lectures, by Brewster, 2 vols. 8vo. Walker's Lectures on Natural Philosophy, 2 vols. 4to. Hauy's Natural Philosophy, translated by O. Gregory, 2 vols. 8vo. Dr. O. Gregory's Lessons in Astronomy, 8vo. Greig's Astrography, or Heavens displayed, 12mo. This is one of the most useful little books extant, for conveying a view of the heavenly bodies, and is illustrated with neat engravings on wood, of the different planets and constellations.

Matthew Young's Analysis of a Course of Lectures on Natural Philoso hy, 8v0. Professor Playfair's Plan of his Course of Lectures, 8vo. The Contemplative Philosopher, 2 vols. 12mo. And the Cambridge Course of Natural Philosophy, by Messrs. Vince and Wood, 4 vols. 8vo.

CHAP. XII.-CHEMISTRY.

THE object of chemistry is to ascertain the ingredi

ents of which bodies are composed-to examine the compounds formed by the combination of these ingredients --and to investigate the nature of the power which occasions these combinations. The word chemistry is supposed to be derived from the Arabic to conceal, and it probably referred to the profound secrecy observed by the old alchemists in their operations.

1. Alchemy took its rise among the Arabians about the commencement of the fourth century. This delusive dream, holding out a bait to avarice, soon attracted a host of followers. Intoxicated with the idea of boundless wealth, they occupied their time in searching for the philosopher's stone, and for a pana, cea, or universal remedy, which should cure every disease, and confer the boon of immortality on the fortunate discoverer of the invaluable secret. Dioclesian, fearful that the visions of the alchemists might be realized, ordered all the books to be burnt. Roger Bacon the alchemist, was excommunicated by the pope, and suffered ten years imprisonment for supposed dealings with the

devil; and Paracelsus was thought to have an evil spirit in the pommel of his sword. The language of this prince of alchemists, was a tissue of boasting and falsehood: he promised immortality in this world to his disciples, but his own death, in 1541, opened the eyes of his deluded followers, and blasted their sanguine hopes.

2. The empire of chemistry has been wonderfully extended within the last half century. It is but a short time since this science recognized, as the subjects of her sway, only a few metals and medicines. She has lately subjected to her sceptre, the various kinds of earths found in the composition of our globe--the different fluids with which we are conversant, whether of the aqueous or gaseous form-the various kinds of vegetable, animal, and mineral bodies which surround us-and almost every substance capable of composition or analysis. In short, she has extended her claims to every species of animate and inanimate matter; and maintains authority over a territory of physical science which may be called immense, when compared with her former dominions. By its ancient cultivators, chemistry was chiefly regarded as an object of curiosity, or as a source of amusement; but in the hands of later chemists it has been converted into a very instructive, interesting, and invaluable science. There is scarcely an art of human life which it is not fitted to subserve scarcely a department of human inquiry or labour, either for health, pleasure, ornament, or profit, which it may not be made, in its present improved state, eminently to promote.

3. To the husbandman, this science furnishes principles and agents of inestimable value; it teaches him the food of plants-the choice and use of manures-and the best means of promoting the vigour, growth, productiveness, and preservation of the various vegetable tribes. To the manufacturer, chemistry has lately become equally fruitful of instruction and assistance. In the arts of brewing, tanning, dyeing, and bleaching, its doctrines are precious guides. In making soap, glass, pottery, and all metallic wares, its principles are daily applied; and are capable of still more useful application, as they become better understood. Indeed, every mechanic art, in the different. processes of which, heat, moisture, solution,, mixture, or

fermentation, is necessary, must ever keep pace, in improvement, with this branch of philosophy. To the physician this science is of still greater value, and is daily growing in importance. He learns from it to compound his medicines, to disarm poisons of their force, to adjust remedies to diseases, and to adopt general means of preserving health.

4. To the student of natural history, the doctrines of chemistry furnish instruction and assistance at every step. of his course; as many of his inquiries can be prosecuted with success only through the medium of careful analysis. To the public economist, chemistry presents a treasure of useful information. By means of this science alone, can he expect to attack, with success, the destroying pestilence, so far as it is an object of human prevention; and to guard against other evils to which the state of the elements give rise. And, to the successful prosecution of numberless plans of the philanthropist, some acquaintance with the subject in question seems indispensably necessary. Finally, to the domestic economist, this science abounds with pleasing and wholesome lessons. It enables him to make a proper choice of meats and drinks; it directs him to those measures with respect to aliment, cookery, clothing, and respiration, which have the best tendency to 5 promote health, enjoyment, and cheapness of living. And it puts him on his guard against many unseen evils, to which those, who are ignorant of its laws, are continually exposed. In a word, from a speculative science, chemistry, during the eighteenth century, has become eminently and extensively a practical one,-from an obscure, humble, and uninteresting place among the objects of study, it has risen to a high and dignified station --and, instead of merely gratifying curiosity, or furnishing amusement-it promises a degree of utility, of which no one can calculate the consequences, or see the end. As it would be quite impossible to give any thing like a representation of the almost interminable field of chemical science, we must content ourselves with a bird's-eye view; and consequently confine our attention to a glance at the following sabjects;-atmospheric air, caloric, water,, the earths, alkalies, acids, salts, simple combus

A A

tibles, metals, oxyds, combustion, attraction, repulsion, and affinity.

I. ATMOSPHERIC AIR, contains oxygen, nitrogen, and carbonic acid gas. 1. Oxygen. If the fresh leaves of plants are placed under a glass vessel filled with water, inverted in that fluid, and then exposed to the sun, they emit bubbles of air, which collect at the top of the vessel, and are found to be oxygen gas, or oxygen in an aëriform state. The same kind of air may be obtained by heating nitre in close vessels, or by distilling the black oxyd or calx of manganese with sulphuric acid or oil of vitriol; the gas, however, is most pure when obtained by the latter process. This gas is absorbed by combustible bodies, and converts them into acids. It is essential to the process of combustion whilst burning; and as it unites with bodies that burn, increases their weight, and changes their properties. It is also essential to respiration, no animal being able to live in air which is deprived of oxygen gas. 2. Nitrogen gas, is unfit to maintain combustion or support life; yet a small portion is absorbed in respiration. It is a little lighter than atmospheric air, uninflammable, and one of the most general elements of animal substances. Though nitrogen gas is, by itself, so noxious to animals, it answers an important end when mixed with oxygen gas, in atmospheric air. Were it not for this large quantity of nitrogen in the atmosphere, the blood would flow too rapidly through the vessels, and would shorten the life of man. 3. Carbonic acid gas is incombustible, and does not detonate with oxygen gas. It is noxious to animals.

The atmospheric air which is produced by this mixture, supports animal life by giving out its oxygen and caloric to the blood. The blood in the veins is purple, approaching to black; but when it arrives at the lungs, it imbibes the vital air of the atmosphere, which changes its dark colour to a brilliant red, rendering it the source of internal heat and animal motion. The loss of oxygen, by respiration and combustion, is supplied by the leaves of trees and other vegetables. They give out during the day a large portion of oxygen gas, which uniting with the nitrogen gas thrown off by animal respiration, constantly