are indebted for all the productions of our fields and gardens, and for all that diversity of prospect and colouring which the vegetable tribes spread over the landscape of the world. It is true, indeed, that water is also necessary for the production of plants. But what is water? It is nothing else than a composition of two kinds of air, oxygen and hydrogen, combined in certain proportions. Now, it is found that plants have the power of decomposing water into these two principles, throwing off a part of the one, and absorbing a part of the other. The elasticity of the air has likewise an important influence on the air-vessels of vegetables; for the contained air, alternately expanding and contracting, according to the increase or diminution of the heat, alternately presses the vessels, and eases them again, thus keeping up a perpetual motion of their juices. It has likewise been ascertained, from recent experiments, that the pressure of the atmosphere has a powerful influence on vegetation, which suggests to us one of those causes which prevent trees from flourishing on the elevated sides of lofty mountains.

2. The pressure of the atmosphere has an influence in preserving water in the state in which we find it. Nothing is of more importance to the comfort of man and other creatures, and to almost all the processes of the arts, than water-without which our globe would be transformed into an immense desert. But, it there were no atmosphere, all the waters on

the face of the earth would boil, and be evaporated with a very slight degree of heat. The ocean would be drained to its lowest caverns, the rivers would cease to flow, the springs would be dried up, and the whole surface of the land exhausted of that moisture so essential to the existence of the animal and vegetable world. Indeed, it is not improbable, that all the substances on the earth, solid as well as fluid, would be dissipated into vapour. That such effects would actually take place, appears from a variety of experiments. If we fill a long-necked bottle with boiling water, and cork it close, so as to exclude the air, and place it in a bason of cold water, the water will sink in the neck of the bottle as it cools. This shrinking of the water will produce a vacuum in the upper part of the bottle, and the water within it will be seen to re-commence boiling with great violence, which can arise from nothing but the cork taking off the pressure of the atmosphere from the water. In like manner, if we place water that has been cooled several degrees below boiling, under the receiver of the air-pump, it will begin to boil as soon as the air is exhausted. It requires a heat of 212° of Fahrenheit's thermometer to make water boil under the common pressure of the atmosphere; but in the exhausted receiver of an air-pump, it boils when heated to only about 67°. The phenomenon exhibited by what is called the pulse-glass, is also owing to the same cause. This glass, having two bulbs, is partly

filled with spirits of wine, the air is extracted, and the glass hermetically sealed; and when the hand is applied to one of the bulbs, it causes a heat which produces an ebullition in the spirits of wine. It is likewise owing to this pressure that porter, ale, and other fermented liquors are preserved in bottles; without which they would either rush with violence out of their mouths, or burst them to pieces. It is owing to the same power that boiling water is preserved in our pots and kettles, when used in cooking, without the influence of which it would soon dilate itself, rush over the vessels, and be dissipated into vapour.

3. It is to the atmosphere we are indebted for the action of fire and flame. Fire is essentially necessary to human existence, even in the warmest climates of the globe. By its means the inhabitant of the desert frightens from his dwelling the beasts of prey, and drives away the insects which thirst for his blood. By its means also, man, in every country, prepares his food, dissolves the metals, vitrifies rocks, hardens clay, softens iron, tempers steel, and gives to all the productions of the earth the form and combinations which his comfort and necessities require. But, without the vital air, no flame can be extricated, nor fire made to burn. This is proved by putting a burning taper within the receiver of an air-pump, and when the air is extracted it is instantly extinguished. The act of combustion effects an analysis of the air; it separates its component

parts: the oxygen of the atmosphere combines with the combustible body, caloric, in the form of sensible heat, is thrown off in every direction; and therefore, where no oxygen exists, it is impossible to make even the most combustible body produce heat or flame.

The

4. It is on the influence of the atmosphere that the respiration of all animals depends. The process of respiration is carried on by means of the lungs. These are distinguished into right and left. The right, or larger lung, is divided into three lobes; the left, or smaller, into two. The internal fabric of the lungs is composed of an infinite number of small membranous cells, full of air, communicating with one another, the number of which Dr. Keil and other anatomists have computed to be at least 1,744,000,000, that is, one thousand seven hundred and forty-four millions. air from without rushes into these vesicles, and is again expelled 1,200 times every hour; and during the same time we consume about 48,000 cubic inches of air, or, at the rate of seventyseven wine hogsheads in a day. The chief uses of respiration are-1, to bring the blood. in contact with the air; 2, to effect certain changes in the mass of the blood; and 3, to produce animal heat. Accordingly, the lungs are so constructed as to allow the largest possible quantity of deteriorated blood to enjoy the fullest intercourse with the largest possible quantity of vital air. It has been calculated by Dr. Hales, that each air cell is the one

hundredth part of an inch in diameter, and that the amount of surface furnished by them, collectively, is equal to twenty thousand square inches. Others have estimated the surface to be more than 1,500 square feet; and Dr. Monro states, that it is thirty times the surface of the human body. From numerous experiments, it has been found that the blood perpetually receives oxygen gas from the atmosphere by the agency of the lungs, and that its red colour is derived from this source. The blood is purple when it arrives at the lungs from the heart; but, having there thrown off hydrogen and carbon, it imbibes the vital air of the atmosphere, which changes its dark colour to a brilliant red, rendering it the spur to the action of the heart and arteries, and the source of motion and of animal heat. The blood is thus indebted every moment to the invigorating influence of the atmosphere, without which the heart would cease to beat, the circulating fluids would stagnate, and the body become a cold putrid mass, without sensibility or motion.

The following are some of the results of experiments in relation to this subject, lately performed by Dr. S. Smith. "1. The volume of air ordinarily present in the lungs is twelve English pints. 2. The volume of air received by the lungs, at an ordinary inspiration, is one pint. 3. The volume of air expelled from the lungs at an ordinary expiration, is a little less than one pint. 4. Of the volume of