attempt to supercede the regular books, but I will endeavour to prepare your minds for the study of them. I will place the subjects in new points of view, and I may sometimes succeed in simplifying them. If I do not add much to your technical knowledge, I will, at least, do something towards the expansion of your views, and "set you a thinking" in such ways as shall raise you a step in general intelligence. Whatever I attempt shall also have clearness and plainness to recommend it; and I will, if you will follow me, be as familiar on paper, as I know you have often considered me in my lectures in the school-room.

In the hope of stealing your attention, I will open the subject in this concluding paragraph, by stating that, in its most extended sense, nature consists of space, or simple expansion; and of matter, within space, which matter, variously compounded and constructed, is called body or substance. Thus the room in which you are consists of space within the walls, the floor and ceiling, and this space itself is filled with matter in various forms of body or substance, as the air which fills the largest portion, the chairs, tables, benches, curtains, and your own persons, your clothes, &c. &c. Now, then, all these substances or bodies, are variously constructed or compounded of matter. Some contain much matter and are solid, heavy, or dense, as the metallic and wooden substances; while others contain little, and are light and rare-some are combustible, others are incombustible-some are mineral, others are vegetable or animal—and it is the business of philosophy to examine their structures, their absolute and relative qualities, their origin, their changes and the causes, and, in short, to answer every rational question that can be put in regard to them. We need, in truth, not go out of the school-room for the materials of instruction which may be easily applied to all the varieties of nature.

LETTER XXXVI.

Properties of the Air.

MY DEAR CHILdren,

You may, perhaps, have judged, that to come into contact with wonders, and find materials for philosophising, it would be necessary to travel farther than your school-room; but on a little attention, it will be found that that room, and your dancing and music-room, contain materials for endless discussion and enquiry. You know that I always disliked the pomp of wisdom, which is usually assumed to serve as a veil to ignorance and imposture. Our knowledge is not of sufficient depth and importance to justify ostentatious displays, for although the studious know much more than the unthinking, yet those who know the most, merely find out how little they really know, and how infinitely much more there is to learn.

The first substance in your school-room, to which I shall call your attention, is THE AIR which fills it. You must be sensible that there is air in the room, and air every where. I drew your attention to it in my observations on music. If you have forgotten them, you should read them again, for however little you may have thought about this air, it is the fountain of your life, and if taken away, you could not live for a moment. Food is the extraordinary and occasional means of supporting life, but air is the ordinary and constant support from moment to moment. It is as necessary to life as the stream of water to a water-wheel, which puts a mill in motion, or as a current of wind to the vanes of a windmill, or the sails of a ship, From the moment we are born till the moment we die, we must take in from fifteen to twenty inspirations of air in a minute, and make as many respirations or expulsions, and the power of doing this constitutes our power of

living. It is just the same with all animal nature, from the lowest to the highest; and it serves as a lesson to pride, that the rich and the poor, those who fare sumptuously, and those who live on loathsome food, must all breathe the very same air or die.

Nor are the powers of this air of which you, and even the philosophers, have hitherto thought so little, confined to animal life, for it is equally necessary to vegetation, and also to fire and flame. Without its presence no vegetable will grow or live, and no fire will burn, or candle give light. To us, therefore, air is in every sense an object of primary consideration.

There is a machine called the AIR PUMP, by which these facts are ascertained as well as many others. By means of it we can withdraw nearly all the air from a close glass vessel, and witness the effects in the vessel when exhausted of air. If an animal, as a cat, or a rat, or a rabbit, be placed in it, and the air then drawn or puinped out, the poor creature will die, unless the air be quickly let in again; and if a candle be placed in it, the flame will be instantly extinguished, proving that the life of one and the burning of the other is entirely owing to the air,

Such being the fact, you will be curious to know what there is in air to occasion such wonderful phenomena. If you turn to your books you will find no information, for air is too familiar a thing to have excited the attention of philosophers more than it did yours before you read these observations. They breathed, and that seemed to be enough for them to know; but the causes and the effects may easily be comprehended by your plain commou sense after reading the illustrations of my two next letters.

LETTER XXXVII.

Mechanical effects of Air.

MY DEAR CHIldren,

Not to know how we live, and why we breathe fifteen times in a minute, when sitting still, and twenty or thirty times when we require more strength and energy, seems to be very disgraceful to our curiosity and intelligence.

I reminded you in my last of two sources of power, in my reference to a water-wheel, and the vanes of a wind-mill. A stream of water moves towards a fall at the rate of three or four miles an hour, or 100 yards in a minute, but on a sudden it arrives at a fall and descends at double the rate, or double the force, and this force it communicates to the vanes, boards, or shoots, of a wheel, which of course it drives round, at the rate of three or four yards in a second. The axle or centre of this wheel is then connected with other wheels, shafts, and contrivances, and made to grind corn, lift weights, and perform all operations which depend on power. In truth, the motion or force of the water is transferred to the wheel, and by the wheel transferred from one part to another till the desired effect is produced.

Just so it is with a wind-mill,-a stream of air moves upon the wanes, forces round the sails, these turn the axle, the axle other wheels, and the resulting force and effects are simply the motion of the air transferred from one part to another.

But mills are contrivances of men, and is it to be supposed that the sublime Author of nature cannot contrive machinery as effective, more subtle, and far more wonderful, than the gross and quickly perishable machinery contrived by man?

In animal machines, in which the motions and functions of the limbs corresponded with the ulti

mate parts of mills; the air is palpably the moving power; but the question is, how does it act,what force is there in it, and how is the force imparted?

It must be evident that if the air consists of atoms, that is, of extremely small particles in great motion one among the other, that if an animal received a volume or measure of them into its chest, and the motions were wholly or partly transferred in the chest, that this would be a source of motion which, constantly repeated, would be equal to the effects of animal strength, motion, and energy. We seem warranted, however, in concluding, that this actually is the means employed, because we perceive that whenever we require more strength, as in lifting or working, or more motion, as in running, skipping, and dancing, we breathe quicker, and take in larger inspirations of air.

All the properties of air correspond too, with the idea that it consists of atoms in great intestine motion; and then if the various kinds of motion are considered which accord with the phenomena, it will necessarily be concluded that they are circular, or in other words, that a volume of air consists of atoms in very quick motion in small circular orbits. But as circular motion is not a natural motion, because all natural motion is straight forward in a right line, so these circular motions arise from the continual striking of the atoms against one another, by which, in purer phraseology, they continually deflect one another into circular orbits. As long as the motions continue, the atoms form what we call air, and though millions may constantly part with their motions to other atoms, yet these in their turn become air, and hence the mass is kept up.

It seems obvious, therefore, that if an animal constantly inspire into his chest and lungs volume after volume of such atoms in motion, and that in the lungs they part with a proportion of the mo