lighter than atmospheric air, called hydrogen gas, the same which, when evolved and excited by heat, causes the oxygen of the air to combine with it, and the union creates light, heat, and what we call fire. The effect, however, is improved when carbon is combined with the hydrogen, as in coals, candles, oil, bitumens, and carburetted hydrogen gas, which is merely this light gas or hydrogen combined with atoms of carbon.

In mines it accumulates and fatally explodes when a lighted candle is brought to it; and in truth every candle and lamp is a similar explosion on a small scale, but with this difference, that the hydrogen explodes as fast as it evolves, but in mines an accumulated mass is exploded at once.

This is worth understanding, for as we are indebted for almost all our enjoyments to light and fire, we ought not to be ignorant, or we are ignorant indeed, that light and fire in every form are merely an operation or process taking place between the oxygen of the atmosphere, and the hydrogen and carbon evolved from a body called, therefore, a combustible. The first effect is produced by an applied heat, as another burning body, and is afterwards kept up by the action and reaction of the three materials as long as they last.

There is no difficulty in all this, except in the terms oxygen, azote, hydrogen, and carbon. But having told you what these mean, I should blush for your understandings if you are puzzled by names. Oxygen is merely that part of the atmosphere which sustains flame, and is necessary to respiration, by carrying off the carbon from the blood in the lungs. Azote is its antagonist and companion in the atmosphere, a sort of diluter, like water to brandy. Hydrogen is the rare gas evolved by burning bodies, and whose uniou with oxygen keeps up the burning. And carbon is the atoms of pure matter, with which oxygen combines both in respiration and burning..

In every case of burning, gas is created, consisting of a combination of hydrogen and carbon, and the only difference between a candle or lamp and a gas-light is this, that in one case the gas is created by the candle or lamp at the wick, and in the other the gas is created at the manufactory, and sent through pipes ready made for use. Every candle or lamp is a gas manufactory on a small scale, and the tallow or the oil contains the fit proportions of carbon and hydrogen gas, calculated, when excited by heat into carburetted hydrogen gas, to combine with the oxygen, always at hand in the air, and their union is heat, light, and fire.

The theory that heat is atoms in motion, and that the transfer of heat from one body to another is the mere transfer of the motion of the atoms of the heated body to the other, enables us to solve all the phenomena of heat and light. Formerly, it was considered that heat was a particular substance or fluid, hot by nature, and called caloric; but as no caloric was ever seen or discovered, only imagined, and as all the phenomena are rationally explained by the theory of atomic motion, the term heat is now properly considered as the name of the effects arising from atoms in motion. Parting with motion or heat, therefore, communicates motion or heat, hence heat diffuses itself from body to body, and hence the great heat in a fire or candle is owing to the atoms of oxygen gas being fixed, or parting with their motion at the burning spot.

Light, as has been stated, is a propulsion of atoms from the burning spot, for as it is supposed that space is full of atoms very near each other, the propulsion of one affects the next just like marbles or billiard-balls laid in rows. Hence light spreads around with such astonishing velocity and such slight force.

Sound is a grosser motion of the mass in a kind of waves, like smooth water affected by a pebble in circular waves.

Animal bodies consisting chiefly of fluids, and kept by respiration at the rate of 80°, 90°, or 100°, the atomic force of that degree of heat necessarily creates an atmosphere of animal fluids intermingled with the peculiar secretions of the animal. Every animal, therefore, has its own atmosphere, extending some feet from its body, and the sense of smelling is in many so delicate and discriminating, that they can distinguish one another at great distances, the power of dogs in this respect being very remarkable.

In concluding this subject I will beg of you to remember, that we breathe twenty times in a minute, and draw in forty cubic inches of air each time; that is, 28,800 times in twenty-four hours, by which we all vitiate as much air as would fill a room ten feet square and seven feet high, every twenty-four hours!

LETTER LVI.

Rain-Snow-Hail-Lightning-Thunder-and Meteors. MY DEAR CHILDREN,

An atmosphere so active, and composed of such antagonist principles, generates constant wonders. When in increased activity, that is, when increased in motion, the motion renders its atoms what we call hot, and it entangles atoms of water, which, like small balloons, rise in the atmosphere and form clouds. This evaporation of course is the greatest in the hottest and finest weather, and is in the year equal to a depth of five or six feet, half of which falls in dews and mists in the evening and night, and the other half in from 24 to 36 inches of rain in these climates, and 70 or 80 inches in hotter ones.

Clouds fall in rain whenever they are suddenly condensed by floating into a cold stream of air, whenever opposing currents of air drive them into collision, and also whenever an electrical action on

their surfaces mingles the atoms or little balloons of vapour, and runs them into drops too heavy to float, and in falling they unite with all the vapours in the atmosphere, and increase in quantity and velocity as they descend.

If the air is cold enough to freeze the vapour, it descends in snow; and under varied accessions of cold they become ice or hail, while the effect in these cases is often produced by electrical action, and hence lightning usually accompanies storms of hail.

But thunder and lightning, the most awful phenomena of the atmosphere, merit special notice. To how many superstitions have these simple phenomena given rise. As I told you in my previous letter on air, electricity consists merely of the mechanical separation of the two components of air-the oxygen and the azote-and the light, fire, violence and phenomena, are caused by large surfaces reuniting through some single point. Clouds present such surfaces and such points; hence if a volume of air is electrically disturbed, and clouds on both sides of it approach, a rapid restoration takes place, we see lightning, we hear its noise in thunder, and the concussion usually causes rain.

Sometimes the earth becomes the medium of communication between two surfaces of clouds, and then there is presented a double stream cf lightning, one downward and one upward, often many miles distant from each other. At other times the earth itself is the opposing surface, and then the strokes are between the earth and cloud. Nothing can be finer than these atmospherical fire-works, and where the effect is between clouds, the horizontal zig-zag, and the magnificent illuminations of the vast vault of the clouds are truly sublime.

In fact, the clouds in this action are vast natural electrical machines, and if you have not a machine, to see sparks of lightning, and hear the explosion of an extensive battery, you may, in a smaller way, imi

tate the whole by rubbing a piece of wax or glas with silk or woollen, and by applying to it in the dark a piece of metal, the light of lightning will thus be made visible, and a slight noise, both of which, on the vast scale of nature, are so grand yet so terrific to ignorance and weakness. The motion of rubbing in this case performs the same office as the motion called heat performs in natural electricity.

But, say you, there is danger in lightning; and so there is, if you come within its line of direction, or its lateral action through conductors. But you are always safe in or upon a bed, if the bed be removed from the walls; or if you sit or stand in the middle of a large room, or in a cellar sufficiently distant from the walls. Buildings too are always protected, which have metallic gutters completely continued into the ground. In the open air you are safe if you avoid the shelter of trees; and when the danger is imminent, if you lie down rather than stand erect. The opposing elements of oxygen and azote (for there is no distinct fluid) obtain reunion by the surface of any non-electric; but the shock over animal bodies destroys life, and the concussion or difficult passage over and through imperfect disunited non-electrics tears them in pieces and disperses the parts in a mo

ment.

The only other phenomena which merit notice on this subject, are the meteors and shooting stars, as they are called. They are, as is supposed, nothing more than bodies floating in space, which the earth encounters in its two motions; and in passing through our rapidly moving atmosphere, they take fire; and getting entangled, often fall to the ground, sometimes as stones, and at other times as gelatinous masses. The notion that they are exploded from the moon, proves that those who write about philosophy, often want common sense, which is the only sense that I apply to these subjects.