gems, diamonds, and gold, in their primitive particles. We will demand of the volcanoes the cause of their central and inextinguishable fires, and the terrific spectacle of their explosions. When the thunder-storm is past, we will sit down with the cottager on the threshold of his peaceful dwelling, inhaling the freshness of renovated nature, and listening to the distant thunder and the renewed songs in the dripping groves, while we gaze in admiration at the splendid bow, which so gloriously spans the heavens from forest to forest, stretched over the dark cloud that has gone by. While we analyze the gorgeous illusion, and learn that it is no more than sun-beams painted upon mist, we will learn the moral lesson to undeceive ourselves in regard to most of the mockeries of the external show of human existence.

Before I commence an exposition of the most obvious laws of the physical universe, and their application to my subject, I shall abridge a sketch of the voluminous works, that present the details of the ancient systems of physics.

The most striking feature of all these doctrines of the philosophy of the olden time is, that they were not so much the study, or the science of nature, as the opinions and theories of the philosophers. Philosophy was not supposed to consist so much in observation of facts, as in speculation upon systems. Every master bore a standard inscribed with the name of his school, and the disciple stood pledged to the system of his master. In physics, no point seemed to them more important than one, which has been a most prolific theme with modern philosophers - the question, how the world was made, and of what elements it was composed? Thales created the world from water. Pherecides held it easier to compose it of earth. Anaximines found air sufficient for the purpose; while Zeno availed himself of the four elements united. It was almost as difficult to people the world as to make it. Other sages followed them, kindling prodigious theoretical subterranean fires, in which grand workshop, they fabricated metals, stones, and earths. The disciples of Aristotle considered air, earth, water, and fire, as the four elements, out of which every thing was composed;

and with these elements, properly moulded and organized, and attempered by the central heat, they held it possible to cover the earth with plants, and people it with men.

If we consult their opinions, in respect to the celestial phenomena, we shall be no less surprised. The sun, which Carsini ascertained to be a million times larger than the earth, Heraclitus affirmed to be no more than a foot in diameter. Even Anaxagoras, a more enlightened astronomer, did not believe it to be larger than Peloponessus. This sublime orb, the rays of which Newton has analyzed, according to Thales, was only an inflamed vapour; or,, in the judgment of Anaxagoras and Democritus, an ignited mass of rock. Before we spend a smile upon these systems of the ancient sages, let us observe the obstacles, which truth has had to surmount in modern times. Let us not forget the reception given to the true system of the universe, as declared by Galileo, the discovery of printing, the circulation of the blood, and, in fact, most of the great discoveries. It is only two or three centuries since physics has become a science of facts, instead of opinions. Torricelli, in ascertaining the weight of the air, and Bacon, in indicating the avenue to almost all the modern discoveries, prepared the way for Newton and Lavoisier. The chief burden of these letters will be the development of some of the views and demonstrations of these great men.

I here present you a tabular view of the chief doctrines and propositions of that branch of physics, which relates to matter and its properties. Large treatises of philosophy have been written upon each one of these propositions. Most of them have been either illustrated or demonstrated, by diagrams and mathematical reasoning, which my plan precludes. Let not the apparent intricacy and hardness of the terms, in which these propositions are declared, alarm or revolt you. It was necessary to conciseness, and to avoid the tedious circumlocution of explanatory verbiage, to include doctrines which involve a vast amount of inferences and details, in abstract terms and the most compact form. If you commit them to memory, you will always have at hand the

chief facts, about which the science of physics is conversant. If hereafter disposed fully to investigate the subject, not only in the principles, but in the illustrations and demonstrations of voluminous books of philosophy, you will find the advantage of carrying to those works the distinct remembrance of these propositions.

A body is a material substance. All bodies' possess extension, form, bulk, divisibility, impenetrability, mobility, gravity, porosity, compressibility, and elasticity. Bodies are solid, as stone; liquid, as water; æriform or gaseous, as air. Many bodies, as water, mercury, and the easily fusible metals, can assume each of these forms, and return, by reversing their transitions, to their original state. There are bodies, which are not seen, except in the form of solids as the infusible metals and the simple earths.*

Bodies have a determinate form in consequence of cohesion and aggregation. The aggregation of bodies is chiefly effected by heat and chemical combination. When two intimately combine, the one often imparts its aggregation to the other. Again the aggregation of the compound is often different from the simples that composed it. Examples. Salt liquifies in water; water becomes an elastic fluid in air. Two gases, muriatic acid gas, and ammoniacal gas, form solid muriate of ammonia. The hydrogen and oxygen gases united, form water.

The changes in the states of the aggregation of bodies probably depend upon the opposition of two forces, one attractive, the other repulsive. The first is an inherent property of matter; the second is produced by caloric. When the attractive force exceeds the repulsive, the body is solid. When both are in equilibrium, liquid; when the repulsive exceeds the attractive, æriform.

Most bodies are composed of heterogeneous substances, as culinary salt, of muriatic acid and soda.

Those

* In indicating temperature, reference is had to Fahrenheit's thermometer. Water exists in the form of a solid at any temperature below 32 degrees; above that, it is liquid; till it reaches 212 degrees, when it becomes ariform or gaseous.

substances, which compose another, are called its constituent principles, to distinguish them from the simple, integrant particles of a homogeneous body.

There are reckoned over fifty bodies, undecomposable by any analysis yet known. This number is continually varying with the varying results of chemical experiment. Most of the substances, formerly considered elements, as air, earth, and water, have been decomposed. Even potash and soda, until very recently considered simple substances, have been found to be metallic oxydes.

The mixture of two substances results from a mutual attraction, or tendency to penetrate into the interstices, or spaces, which exist between their particles. This tendency is called their affinity. We know nothing of matter, but its. properties. There are, however, two prevalent theories to explain its intrinsic nature. The first, called the atomic theory, prevails in France. According to this theory, matter is composed of indivisible, impenetrable particles, called atoms. These atoms are kept apart by attractive and repulsive forces, and porosity results, as a necessary property. There is more void space than matter, in every body. The varieties of bodies result from a material difference in these atoms, or in their form, magnitude, position, and distance.

The dynamic theory considers matter as filling the space occupies. According to this, porosity is an incidental property; but compressibility and dilatability essential properties. The state of the body depends upon its attractive and repulsive forces. Change their ratios, and its volume changes. There are certain primitive substances, the varirious combinations of which produce all bodies. This is the theory of Germany. Neither is susceptible of proof. That philosophy, which admits nothing as certain, but what has passed the test of experiment, is called the inductive philosophy, and is the true mode of philosophizing.

Motion is change of place, and is rectilinear, or curvilinear, varied, accelerated, or retarded. The space passed over in a given time, by a body in uniform motion, is called

its velocity. In uniform motion, the space described is as the time. In compound motion, if a body receive an equal compound force acting in rectangular directions, the body will describe the diagonal, in the time that is requisite to describe either of the sides, with the simple force acting in either direction. This is a difficult law of motion to illustrate, except by presenting a diagram; but it is the most important doctrine of rectilinear moving forces. By this law, any number of motions may be compounded into one; or a single motion may be decomposed into any number of constituent motions.

The moving force of animated beings is communicated by volition; of inanimate bodies by impulse. A body at rest, without impulse, remains at rest. A body in motion continues to move in a right line, until the motion is changed or arrested by some extrinsic moving force. This indifference of matter to motion or rest is called the force of inertia. A force is measured by the product of the mass multiplied into the velocity of the moving body. The action and reaction of two bodies upon each other are always reciprocal and equal. In other words, if two moving bodies meet, the motion, which the one gains by impulse, the other loses.

But the law of matter, which far surpasses all others in its important relations to physics, and in the magnitude of the discoveries to which the knowledge of it has led, is the gravity of attraction. It acts at vast distances, sustains the worlds in space, and determines the extent and form of their orbit round their suns. It is this, which unites, in one vast whole, all the bodies, of which the planetary system is composed, imparting perpetual order and harmony to their motions. If the Creator were for a moment to suspend this universal power, all nature would return to chaos.

Gravity is the reciprocal attraction existing between all the bodies in nature, operating in the direct proportion of their masses, and in the inverse proportion of the squares of their distances. The measure of this is called weight; and, in homogeneous bodies, is as their bulks. Specific gravity