tion, and the volumes when expelled possess less elasticity or motion, that the obvious use of breathing or of air, is to impart motions to the animal system, while such motions directed towards and by the limbs are the sufficient immediate cause and causes of their powers.

The air in the school-room is not, therefore, a quiet mass of atoms, but a mass of millions of millions of atoms in great and constant circular motion, without which it would not be air; and the lungs of animals are the sublime contrivances of the allwise Creator and Preserver, by which these motions are imparted to the animal frame. The importance of a supply is even apparent in the projecting nostrils of all animals, for even our noses, of the fine forms of which many of us are so weakly vain, seem to be but copings by which the air is more effectually assembled and carried through the wind-pipe to the lungs.

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LETTER XXXVIII.

Nature and Phenomena of Heat,

MY ESTEEMED Children,

It will be necessary before we proceed further in these enquiries, to consider the nature of the effect which we call HEAT, by means of which solids are converted into fluids, and fluids into gas, or air; and which also displays itself in some bodies in the beautiful and useful forms of fire, flame, and light. You are under so many obligations to it, and it it so connected with your being and existence, that I am persuaded you will follow me with the same patience as that which you displayed in reading the previous letters, and on my part I will endeavour to make it as plain and familiar as possible.

If you strike or rub your hands together you feel

heat; if you strike them on the table you feel the same; if you rub two sticks together they become hot, and will in time take fire; if you strike a nail several times with a hammer it will burn you; if you bore a hole with a gimblet or augur it will become intensely hot; in short, if you produce any kind of quick definite motion, heat is the result, and heat is, therefore, the excited and accumulated motion of atoms, and all its phenomena may be explained by the sufficiently obvious fact, that heat or its power is felt or seen whenever atoms are excited into great motions, and they impart their motions to other atoms. In brief, motion parted with by one set of atoms to another set, creates in the new set, and in the body with which it is connected, the sense and phenomena of HEAT, which, in all its various and complicated forms of display, is merely atoms in motion, or atoms in motion transferring their motions. Keep this in view, and as the accession and dispersion of heat or atomic motion, occasions most of the changes which occur in natural bodies, so the causes of those changes may, by this simple principle, be easily explained.

If I strike a nail with a hammer, and on touching it my finger is burnt, the cause is as follows: the parts or atoms of the nail were put by the blows into intense motion, and as my finger would on touching it partake of the excitement of the nail, the fluid matter in the skin would be instantly dispersed by the motion, and the skin being dried and contracted, would press and injure the nerves, and thereby cause pain.

But if I had put a little water on the nail struck by a hammer, the excitement of the nail would be transferred to the water, by which the nail would be cooled, but the blows of the hammer would then be transferred to the atoms of the easily divided water, and the water would become steam, gas, or a species of air, the volume or bulk of which would

be proportioned to the first excitement, such, in fact, being the general nature of all steam and all air. The atoms as they rise or evolve would be successively driven into circular orbits, because the space into which they rise was previously filled with aerial atoms; and each rising atom would be acted upon just as a billiard ball would be that might be propelled on a billiard table on which many balls had previously been laid. The re-action of the several balls would turn it into a circle, the size of which would depend on the force of propulsion; and the analogy explains the expansion, the compression, and other properties of gas and air: for a steam engine, and the great laboratory of nature, act on the very same general principle.

All the phenomena dependent on heat or motion. in the atoms of the atmosphere or air in the schoolroom, are, however, much varied and complicated, by the fact, that this air is composed of different atoms, but in what they differ, or how they move in mutual connexion, must for ever, perhaps, be unknown. They produce, however, many different effects; one set being of an acid character, and the other alkaline; the acid set being likewise the supporter of animal life and flame, while the other set are adverse to both. The first is called OXYGEN, or the acidifying principle; and the other NITROGEN or AZOTE, or, in a general or contrasted sense, the alkaline principle. Their proportions in the atmosphere are as one of oxygen to nearly four of nitrogen, and there also floats with them about a 40th part of aqueous and other gases. The contrasted action, or the mutual action and re-action of these intermingled gases, and their antagonist powers, when they are separated, give rise to the silent but persevering activity of nature.

There is also a much rarer gas, called hydrogen, 15 parts of which, with 85 of oxygen, form water. It also constitutes oils and unctuous bodies, and the

disposition of oxygen to unite with it, when excited, is so great, that when hydrogen is evolved from unctuous bodies by the previous application of great heat, the oxygen flows in from the surrounding space, and parting with its motions, these produce further heat; and a continued flame is kept up, as long as there is any hydrogen to evolve and oxygen present to flow in. This is the phenomena of a common candle, a coal fire, a gas light, &c. &c. All these curious and useful things are mere conflicts of hydrogen and oxygen under different cir

cumstances.

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LETTER XXXIX.

Elementary, Principles.

MY ESTEEMED Pupils,

You perceive, then, what an extensive theme is the air of your school-room, about which, I suspect, you never thought before. You never thought of it as full of motion and power, as the source of your motions and powers, as a compound of two antagonist principles, whose opposing energies are always creating and destroying, nor that one of these principles, by combining with a third, was the cause of fire and flame.

We must not proceed too fast, for we have another subject to carry into our reasonings. Whatever has power must be matter in motion; and, therefore, the gases described must consist of atoms, and as different, so of different atoms in motion. Of their different forms or structures no guess can be made, because they are as much smaller than a grain of sand, as a grain of sand is smaller than a block of Portland stone, for in nature, there can be no limit either to smallness or vastness. There is, however, a neutral set of atoms, neither oxygen, nitrogen, nor

in

hydrogen, all which, probably, derive their power from the smallness of their atoms, and their consequent facility of motion. This neutral substance is called CARBON, and you must not be frightened at an unusual word, for it means neither more nor less than mere atoms of simple substance which combined with Oxygen, Nitrogen, and Hydrogen in various proportions, forms nearly all kinds of bodies. Oxygen, fixed with metals forms oxides, or rusts, or calces; and it is believed that all earths are mere oxides; it also constitutes all acids. Nitrogen or azote, in its fixed state, constitutes the chief part of animal and vegetable substances. Hydrogen, in its fixed or solid state, is also found in animal and vegetable bodies, and it confers their characters on oils, fats, and spirits of all kinds. But the universal constituent of bodies, combined with the others, is Carbon, and it is so universally combined that it is said to be only pure the diamond; though, even as the diamond can be burnt, it would thence appear, that it contains hydrogen, because all burning, as I said before, is the union of oxygen with evolved hydrogen, or of the same principles, under whatever form they appear. But we have not done with the air of the schoolroom, which has many curious properties besides those already described. I dare not attempt to follow the whole, and therefore refer you to your Watkins, or Grammar of Philosophy. Imust, however, mention one quality, on account of its astonishing results. As the different atoms are differently susceptible from the same action or excitement, and similarly affected by different excitements, so they may, in certain cases, be separated within the same space, or between boundary surfaces. But as union is their fit and natural state, so, when separated, reunion is sought by each with a certain degree of force. This separation of the two great principles, the acid or alkaline, the force with which they endeavour, when separated, to reunite, the phenomena which at