fact throughout nature, will be apparent at a future Season.

Solids, liquids, and aëriform substances, though of the same thermometric temperature, have different capacities for heat; and these are most singularly affected by changing their physical state of density.

Thus, at common temperatures, if a given bulk of Water weigh 1000 parts, the same bulk of Lead will weigh 11,350 parts; if this be violently compressed by machinery, the individual particles of the metal are thrust, or packed into less bulk than they originally occupied, the lead becomes more compact or dense; and now, if its weight be compared with that of an equal bulk of water, it may be found perhaps increased to 11,360 parts, or, in other words, the lead has increased in density.

Lead, in common with other substances, contains a certain amount of heat, or has a certain capacity for heat; and if a strip of the metal, placed on an anvil, be suddenly and forcibly beaten with a hammer, its density is increased, but its capacity for heat is diminished; the particles of the metal being compacted into a smaller space, have not room for all the original heat; some, therefore, escapes, and warms the lead, to a degree painful to the hands; indeed, it will immediately kindle a small piece of phosphorus.

A dextrous blacksmith will place the point of a soft iron horse-shoe nail upon an anvil, and strike it only two violent blows with a hammer, and the point will instantly become red-hot, and inflame a sulphur match; thus he generally obtained fire, before the introduction of Lucifer matches.

In further elucidation of these curious matters,

which at first sight may appear to have no connection with the chemical phenomena of the Four Seasons;-a piece of sponge may represent the metal; the sponge has a certain capacity for holding water, the same as the metal has a certain capacity for holding heat; when the sponge is squeezed, its particles are brought closer together; it has less capacity for holding water, and accordingly some will run out; so the metal, when struck, has its particles more closely compacted, and therefore less capacity for holding heat, and some must escape.

Change of density in liquids is also attended with change in their capacities for heat, and the chemist can furnish experimental illustrations of this phenomenon ;provide an apparatus resembling the annexed engraving.

Fig. 24.

It is a glass tube, twelve or fourteen inches long, half an inch diameter, closed at one end, and enlarged at the other into two bulbs, about three inches diameter, connected by a neck the same size as the tube; the bulb farthest from it has a mouth, provided with an accuratelyfitted stopper, slightly touched on its ground part with pomatum, for a reason already described.

Remove the stopper, and holding the apparatus upright by the bulb to which the stopper belongs, pour in water until the tube and lower bulb are filled; then pour in alcohol, until the upper bulb and its mouth are filled; carefully insert the stopper, and wipe away the little quantity of alcohol that will flow over.

In this arrangement, the water, being the heaviest

liquid, will remain below, and distinct from the lighter alcohol; the two liquids are prevented from mixing rapidly, by the narrow neck between the bulbs, and they are both of the same thermometric temperature.

Place the palm of the hand over the stopper, grasp it and the bulb, and invert the apparatus, that the tube may now stand perpendicularly; and in this position the heavy water will fall through the light spirit, or vice versa; the tube will not appear full as it was originally, and the bulbs will become sensibly warm.

Why is the tube deficient of its contents, for the ground stopper has prevented any portion of liquid from escaping, and why do the bulbs feel warm?

Because the light alcohol, by combining with the water has its density increased, it occupies a smaller space than at first, and, like the solid lead, has its capacity for heat diminished; a portion therefore escapes, and affects the hand.

These experiments are preliminary to others upon the atmosphere, which most concerns our present inquiry; and we shall find that changes in its density, by condensation and refraction, produce changes in its temperature, elevation in one case, depression in the other.

All aëriform substances admit of more sudden condensation and rarefaction than either solids or liquids, and therefore experiments, upon such highly attenuated and often invisible forms of matter, are not only extraordinary, but instructive.

The annexed sketch represents an apparatus for proving that the condensation of air compels it to part with heat.

Fig. 25.

It consists of a well-annealed, and perfectly cylindrical glass tube, fourteen inches long, a quarter of an inch thick, and rather more than a quarter of an inch internal diameter, containing air; one end of this tube is closed with resinous cement, which likewise secures a brass cap, terminating in a ball, about one inch and a half in diameter, whilst the other end of the tube is open, but its circumference bound with a brass band, about two inches broad, to prevent it from being accidentally chipped.

The other part of the sketch represents a rod of welltempered steel, about one inch longer than the glass tube, but rather smaller than the bore, for about twelve inches; one end of this rod is firmly screwed into a flat brass knob, about one inch in diameter, whilst the other end is enlarged and closely packed with oiled leather, to form a piston, which accurately fits the tube; the end of the piston has a small hook, or it is slightly cupped, to hold a fragment of perfectly dry German tinder, about the size of half a pea.

Insert the piston with the tinder into the glass tube; place the brass knob upon a solid table, that the brass ball may be uppermost; grasp this with the right hand, steady the tube with the left, then with one sudden and powerful jerk, thrust the tube down as far as practicable; a vivid flash of light will instantaneously appear; and upon quickly withdrawing the piston, the tinder will be found burning as perfectly as though it had been kindled at a common flame.

Why is this? Because the sudden compression, condensation, or increase of the density of the air in the tube, diminished its capacity for heat, or, in other words, the sudden thrust beat the heat from the air, as the sudden

blow beat the heat from the lead in the first experiment; the heat is caught by the tinder with sufficient intensity to cause its combustion.

If the experiment do not succeed, the piston must be withdrawn, the tinder examined, and, if oily, it must be thrown away, and replaced by a fresh dry piece ;-the air in the tube must be renewed by introducing a long straw, applying the lips to its projecting end, and gently drawing, not blowing, fresh air through for three or four seconds; then the experiment may be tried again, and, after a few trials, the proper method of sudden condensation will be discovered, so that success will be certain at any future time.

If so much heat be elicited from so small a portion of air as that contained in the glass tube, what an inconceivably enormous store must be emitted from the vast atmosphere by natural changes in its density throughout the Four Seasons.

Diminution of the density of the air, or, in other words, its sudden rarefaction or expansion, produces the opposite effect of cold; an experiment in proof of this fact may be performed by the aid of an air-pump, and most