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flying out before it, both in its annual and diurnal course.

In short, it appears not altogether improbable to suppose, that the visible universe is filled with some fine elastic fluid or air, but of such a rarity as to be no sensible hindrance to the celestial orbs in their rapid motions through the regions of space; and that this fluid accumulates itself around every planetary body, in proportion to the quantity of matter it contains—the larger bodies attracting more of it, and the smaller bodies less; and thus forming an atmosphere around each, corresponding to its nature and destination. And, if this be the case, the atmosphere of the earth can have no definite boundary, but may be said to mingle with the atmospheres of all the other planets which belong to our system. There is a certain portion of atmospheric air, however, which must always be considered as attached to the earth, and which revolves with it in its diurnal rotation, and is carried along with it in its course round the sun. If the atmosphere did not revolve along with the earth, we should constantly experience an easterly wind, blowing with an immense velocity of more than a thousand miles an hour, which would produce a most tremendous hurricane, which would level with the ground houses, trees, forests, and every prominent object on the surface of the earth. But the particular region where the motion of the atmosphere terminates, it is impossible for us to ascertain.

CHAPTER VI.

The composition of the atmosphere. For a long series of ages, air was considered by philosophers as one of the four elements of which all things are composed, the other three being fire, earth, and water. But the discoveries of modern chemistry have fully demonstrated that all these are compound bodiesthat even the air itself, fine and invisible as it is, is not a simple substance, but compounded of different ingredients. This is one of the most curious and interesting discoveries of modern times ; and little more than seventy years have elapsed since it was first surmised that the atmosphere is not a simple and homogeneous, but a compound fluid. The experiments which led to this discovery were first made by Dr. Priestley, on the first of August, 1774, on which day he obtained what was then termed dephlogisticated air, now known by the name of oxygen gas, and which forms one of the constituent principles of atmospheric air. It was also discovered in the year 1775, by M. Scheele, a Swedish chemist, without

any previous knowledge of what Dr. Priestley had done, and he gave it the name of empyreal air, from its powerful influence in supporting flame. But, instead of entering into the history of such discoveries, and of the processes by which they were made, a few of the properties possessed by the different ingredients of which our atmosphere is composed may be simply stated.

The air of the atmosphere, then, is found to consist chiefly of two very opposite principles or fluids, termed oxygen gas, and nitrogen gas, along with a very small proportion of fixed air, or carbonic-acid gas.

If any portion of the atmosphere, such as the air in our apartments, be supposed to be divided into an hundred equal parts, twenty-one of these parts will be oxygen gas, about seventy-eight nitrogen, and a hundredth part, or, according to some chemists, a thousandth part, will be fixed air, or carbonic-acid gas. In the first place, a few remarks shall be offered on the nature and properties of oxygen gas.

This gas, like common air, is colourless, invisible, and elastic, and capable of indefinite compression and expansion. Its peculiar and distinguishing properties are :- 1st. It is essential to combustion, and is the only principle with which we are acquainted by which flame can be supported. When acting by itself, it produces the most rapid conflagration of all combustible substances. If a lighted taper be let down in a jar of oxygen gas, it burns with such splendour that the eye can scarcely bear the glare of light, and, at the same time, produces a much greater heat than when burning in common air. If a piece of iron wire, a watch-spring, or a steel file, armed with a piece of wood, or phosphorus, in an inflammable state, be put in this gas, the steel will take fire, throwing out sparks, and producing the most brilliant appearance, almost dazzling the eye with their splendour. In the next place, it is essential to the support of animal life ; for it has been proved by many experiments, that no animal can exist for a single moment in any kind of air which does not contain a certain portion of oxygen; so that man, and all the other ranks of animated beings, may be said to depend upon this substance, not only for their comforts, but for their very existence.

Again, the basis of oxygen gives the acid character to all mineral and vegetable salts, from which property its name is derived; for the term oxygen literally signifies, the generator of acids. In short, oxygen is the vehicle of heat to the animal system-it imparts the red colour to the blood in its passage through the lungs-it constitutes the basis both of the atmosphere which surrounds the earth, and of the water which forms its rivers, seas, and oceans; for water is found to be nothing else than a combination of two kinds of air, oxygen and hydrogen gas. It pervades the substance of all the vegetable tribes, and enables them to perform their functions. In combination with

the different metals, it serves the most important purposes in the useful arts; and, on the whole, may be considered as the most extensively useful, and the most powerful and energetic agent in the system of nature.

Oxygen gas may be procured from a variety of substances, particularly from nitre, manganese, and the red oxyde of mercury, and also from vegetables immersed in water, and exposed to the solar rays. It is heavier than common air, nearly in the proportion of eleven to ten; one hundred and sixteen cubic inches of oxygen are found to weigh about thirtynine grains, while the same quantity of common air weighs only thirty-five and a half grains.

One of the most extraordinary effects of oxygen appears when it is combined with nitrogen in a certain proportion, so as to form what is commonly called nitrous oxyde. This gas consists of sixty-three parts nitrogen and thirty-seven parts oxygen. When it is put into a bladder, and inhaled into the lungs, by means of a pipe, and shutting the nostrils, it produces an extraordinary elevation of the animal spirits-involuntary muscular motion --a propensity to leaping and dancing-involuntary bursts of laughter-a rapid flow of vivid and agreeable ideas, and a thousand delightful emotions, without being succeeded by any subsequent feelings of languor or debility. When Mr. Southey, the poet, inhaled this gas, he declared that it produced in him sensations perfectly new and delightful. His

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