AN OSTRICH.

Obs.-In Art. 127 and 128, were described the transformations of insects, from the egg to the worm; the worm to the chrysalis and the chrysalis to the butterfly.

The following cut represents those four states in the common caterpillar:

THE EGGS, CATERPILLAR, CHRYSALIS, AND BUT

XX. Chemistry.

499. The principal object of chemistry is to ascertain the elementary or first principles, of which bodies are composed.

The ancients conceived that there were but four elements, or first principles;-Air, Water, Earth, and Fire: of these, and by their mutual action, they conceived that every kind of matter was composed.

Modern experimental philosophy has, however, analysed three of these elements, or has discovered other elements of those elements; and until these may have been analysed by further experiments, we must consider them as elementary bodies.

Obs.-SIR HUMPHREY DAVY, in the preliminary observations to his Elements of Chemistry, beautifully observes, that "the forms and appearances of the beings and substances of the external world are almost infinitely va rious, and they are in a state of continued alteration." The whole surface of the earth even undergoes modifications, Acted on by moisture and air, it affords the food of plants; an immense number of vegetable productions arise from apparently the same materials; these become the substance of animals; one species of animal matter is converted into another; the most perfect and beautiful of the forms of organized life, ultimately decay, and are resolved into inorganic aggregates; and the same elementary substances, differently arranged, are contained in the inert soil; or bloom, and emit fragrance in the flower; or become in animals, the active organs of mind and intelligence. In artificial operations, changes of the same order occur; substances having the characters of earth, are converted into metals; clays and sands are united, so as to become porcelain; earths and alkalies are combined into glass; acrid and corro

sive matters are formed from tasteless substances; colours are fixed upon stuffs; or changed; or made to disappear; and the productions of the mineral, vegetable and animal kingdoms, are converted into new forms, and made subservient to the purposes of civilized life. To trace, in detail, these diversified and complicated phenomena, to arrange them, and deduce general laws from their analogies, is the business of Chemistry.

500. It is now found, that the AIR which we breathe is composed of a mixture of two distinct elements; one called nitrogen or Azote; the other Oxygen; and both are kept in their gazeous state by Heat, called Caloric, and that WATER is a mixture of Oxygen with Hydrogen ;-that EARTH is a mixture of many substances;-and that FIRE is composed of Heat (or Caloric,) and Light, united to a combustible substance.

Obs.-The forms of matter are well arranged into four distinct classes, by SIR H. DAVY. The first class consists of solids; which compose the great, known, part of the globe. Solid bodies, when in small masses, retain whatever mechanical form is given to them; their parts are separated with difficulty, and cannot readily be made to unite after separation; some solid bodies yield to pressure, and do not recover their former figure, when the compressing force is removed, and they are called non-elastic solids; others, that regain this form, are called elastic bodies. Solids differ in degrees of hardness; in colour, in degrees of opacity or transparency; in density, or in the weight afforded by equal volumes; and when their forms are regular or crystalized, in the nature of these forms.

The second class consists of fluids; of which there are much fewer varieties. Fluids when in small masses, assume the spherical form; their parts possess freedom of motion; they differ in degrees of density and tenacity; in colour and degrees of opacity or transparency. They are usually regarded as incompressible; at least a very

great mechanical force is required, to make them occupy a space perceptibly smaller.

Elastic fluids or gases, the third class, exist free in the atmosphere; but they may be confined to solids, or by solids and fluids, and their properties examined. Their parts are highly moveable; they are compressible and expansible; and their volumes are inversely, as the weights compressing them. All known elastic fluids are transparent, and present only two or three varieties of colour; they differ materially in density.

Besides these forms of matter, which are easily submitted to experiment, and the parts of which may be considered as in a state of apparent rest, there are other forms of matter which are known to us only in their states of motion when acting upon our organs of sense, or upon other matter, and which are not susceptible of being confined. They have been sometimes called ethereal substances, which appears a more unexceptionable name than imponderable substances. It cannot be doubted that there is matter in motion in the space, between the sun, the stars and our globe; though it is a subject of discussion, whether successions of particles be emitted from these heavenly bodies; or motions communicated by them, to particles in their vicinity, and transmitted by successive impulses to other particles. Ethereal matter differs, either in its nature, or in its affections by motion; for it produces different effects; radient heat, and different kinds of light.

501. CALORIC is a mere name of that element or property, which, combined with various bodies, produces the sensation of heat while it is passing from one body to another.

According to its quantity in different bodies, it renders them fluid; or converts them into gas, or air.

Ice is water deprived of its caloric: when the caloric returns, the ice is again converted into