to see the grass on sandstone districts parched and browned, whilst that on limestone districts is luxuriant and green.

The student of Nature must bear in mind, that the enormous amount of fifty-four trillions, four hundred and fifty-nine billions, seven hundred and five millions. of tons of watery vapor, is sustained by the atmosphere, which is a mechanical mixture of the elements Oxygen and Nitrogen, and the compound Carbonic Acid; and that during all the wonderful vicissitudes of the Four Seasons, the atmosphere is never sensibly altered in the due proportion of its constituents.

Even when replete with solar heat, gained indirectly by convection from the warm surface of the earth, the air is simply expanded in volume, not altered in actual constitution; and when from local causes it has parted with a certain degree of heat, or become cold, it is simply contracted in volume; the pure abstract effect of artificial heat and cold upon air, is likewise attended by similar phenomena; this can be experimentally proved by the following arrangement:

A Florence flask is placed with its neck dipping beneath the water contained in a glass; the flask, although empty, in the ordinary acceptation of the term, is actually full of air; this has no tendency to escape,

because it is nearly of the same density and elasticity as the external atmosphere; neither has the water a tendency to enter but a very little way into the neck of the flask, because it is opposed by the included air.

Apply the flame of a spirit-lamp very gently to the bulb of the flask; its thin glass immediately conducts the heat to the air, which expands in volume, or becomes too large for the flask to contain, and accordingly a portion escapes from the neck, and bubbles through

the water.

Allow twelve or fourteen bubbles to escape, then remove the lamp, and as the flask cools, water from the

glass will gradually rise into the neck of the flask, and at last become stationary.

The explanation of this experiment is as follows:the air was expanded by heat, and a portion escaped; the remainder then contracted by cooling; and if the neck of the flask had not been under water, the external

air would have entered to restore the original volume; but being thus placed, the pressure of the external air forces water upward, to supply the place of the air that escaped.

Again apply the flame of the lamp to the bulb of the flask, until the expansion of the portion of air that it now contains, forces down the water nearly to the neck; then remove the lamp, and upon cooling the water rises as before; again heat and again cool, to the same degree; the air will invariably expand in the one case and contract in the other, and cause the water to fall and rise as many times as may be thought proper; the same results will ensue upon alternate exposure of the flask to sunshine and shade.

When the air has contracted for the last time, lift the flask with one hand, that its mouth may be only half an inch beneath the water, and take a cork between the fingers of the other hand, and dip it beneath the water, to close the mouth of the flask; then it may be placed to stand upon a wooden or pasteboard ring, as shown in the next engraving; the water that was in the neck of the flask, will now occupy part of its bulb; the air will occupy the remainder.

Provide a small wax taper, about one inch and a half long; attach it to the end of a bent wire, rather exceeding the height of the flask; kindle the taper, remove the cork, and gently let the taper down the neck into the bulb of the flask, the flame will be found to continue for a time, with the same brilliancy as in the open air; at last it is extinguished.

These experiments are to prove that the pure and abstract effect of heat from the flame of the spirit-lamp conducted through the glass, is merely to cause expansion of the volume of the air, and that of cold, its contraction; and that no chemical change in the proportion or nature of its constituents is effected, or the flame of the taper could not exist for a moment when plunged into air subjected to these ordeals.

Whether the air be hot or cold, it invariably contains the relative proportions of nitrogen and oxygen, which have been so frequently named; the latter element supports the flame of the taper for a time, but why does it not continue as it would do in the surrounding air? Because actual combustion in air effects its decomposition; chemical changes are instantly summoned into activity; the chief elements of the wax taper are Hydrogen and Carbon; at an elevated temperature both combine with Oxygen, to form in the one case Water, and in the other Carbonic acid; and the flame or chemical change ceases when the given volume of oxygen is thus abstracted, because these results of combustion, water and carbonic acid, and the liberated nitrogen, can yield it no support.

The flame of the spirit-lamp employed for heating the flask, decomposed the free air for its support; and in every case of the combustion of ordinary fuel, the air

is similarly decomposed, the elements of the fuel combining with the oxygen, and producing water and carbonic acid, and liberating nitrogen, for which they have no attraction.

Animal respiration effects changes upon atmospheric air, closely analogous to those of combustion; air that has passed from the lungs is rendered perfectly unfit for the further support of life or flame; the fact as regards flame may be experimentally proved by a very simple arrangement of apparatus, of which a sketch is given

below.

Provide a glass vessel, in the form of a quart bottle, having the lower part removed; withdraw the groundglass stopper, and then place the glass, to stand in a shallow pan of glass or earthenware, containing about two pints and a half of water; introduce a lighted taper through the neck of the glass, and ascertain that the included air supports flame; then remove the taper, lift the glass perfectly out of the water, and move it backwards, and forwards in the open air, to allow the products of the flame of the taper to escape; the glass will thus become filled with fresh air, and is to be placed in the water as first directed.