How are waters and other fluids preserved to us by the weight of the atmosphere?

The weight of the atmosphere, pressing on the water, binds it down as it were, and prevents the usual heat of the sun from converting this and all other fluids on the face of the earth into vapoura.

ment, and shows that a very small degree of heat would be sufficient to evaporate most of our fluids if we had no atmosphere. This instrument is also calculated to show that evaporation produces cold;. for the instant that the spirit begins to boil, a sensation of sudden cold is felt on that part of the hand where the bulb rests.

It is the principle of evaporation producing cold that occasions the injury which persons sustain by sitting in wet clothes. In these circumstances, it is not the water that hangs upon them which produces the mischief, but the sudden loss of a large portion of caloric, which is carried off from the body by the evaporation of this water. If a healthy person were closely covered up with his wet clothes, so that no evaporation could take place, he would probably sustain no injury.

a That the waters on the face of the earth would be dissipated in vapour by a small degree of heat, if we had no atmosphere, may be shown by the following simple experiment:Procure a bottle with a very long neck, fill it with boiling water, and cork it close so as to exclude the air. Then if it be put to stand in a bason of cold water, the water will sink in the neck of the bottle as it cools. This shrinking of the hot water will produce a vacuum in the upper part of the bottle, and the water within it will be seen to recommence its boiling with great violence. This can be owing to nothing but the cork taking off the pressure of the atmosphere from the water. In like manner, water which has been cooled many degrees below boiling will begin to boil again if placed under the receiver of an air pump, as soon as we begin to exhaust the receiver of its air. Under the pressure of the atmosphere water boils at 212°, but in vacuo it boils when heated only to 67°. On the contrary, if additional pressure be given to water by a Papin's digester, it may be heated to 400°, without ebullition. Lead has often been melted by the water heated in these digesters.

At the boiling fountain in Iceland the water is thrown to the height of 90 feet, and is still boiling-hot when it falls to the ground. This water, therefore, must have been much better,

E

How does the weight of the atmosphere operate

so as to be beneficial to the animal and vegetable creation?

The uniform pressure of the atmosphere on the exterior of all organized beings counterpoises the internal pressure of the circulating fluids, and preserves the vessels in due tone and vigoura.

in its reservoir, than the boiling point of water. See Troil's Account of Iceland.

Some philosophers have asserted that, if atmospheric pressure were entirely removed, all substances on the face of the earth, solid as well as fluid, would be dissipated in vapour.

If a small metallic cup be half filled with good ether, and placed within a large watch glass half filled with water, and both be put under the receiver of an air pump, when the air is exhausted the ether will boil, and the water will be frozen, The cause of these effects may be thus explained:-When the pressure of the atmosphere is removed by the air pump from the surface of the ether, its own latent caloric occasions its expansion, and, absorbing caloric from the water, it becomes converted into gas; and the water, having lost its caloric of fluidity, becomes ice.

The best elucidation of the nature of the pressure of the atmosphere which I have seen, is in "Brisson's Physical Principles of Chemistry," section 31 and the following.

a Were it not for the pressure of the atmosphere constringing the vessels in men and vegetables, the elastic fluids contained in the finer vessels would inevitably burst them, and life become extinct. Count Zambeccari and his companions, who ascended with a balloon to a great height on the 7th of November 1783, found their hands and feet so swelled, that it was necessary for surgeons to make incisions in the skin. They ascended to so great a height, that the pressure of the atmosphere was not suffi cient to counterbalance the pressure of the fluids of the body.

Persons who have delicate constitutions need not wonder that they are generally much affected by a change in the atmosphere, when they learn that often in the course of a few hours the atmospheric pressure, on each individual, is increased or diminished from one hundred weight to half a ton weight: while the internal pressure of the circulating fluids remains the same. Supposing a man's body to contain 15 square feet of surface, (which is near the truth,) he will sustain a body of air upwards of fourteen tons weight. But it is necessary to remark that the air presses upwards, downwards, and sideways, in every

What is the weight of the atmosphere?

Each square foot of the earth's surface sustains about 2160 pounds of atmospheric air. A column of air an inch square weighs about 15 pounds.

What other advantages do we derive from this immense atmosphere?

The great thickness of the atmosphere gives a proper temperature to the rays of the sun: it also reflects those rays so as to give a lucid brightness to every part of the heavens; and is the cause

direction; and that it is owing to this equal pressure that we are not injured by the vast weight of the atmosphere; for the equal pressure on all sides resists as much as it is resisted.

Whenever I hold my hand out in this fluid, I feel no weight upon it, because the pressure under and above my hand is equal but if I lay my hand on a hollow cylinder of glass, placed on the plate of an air pump, and exhaust the air out of the cylinder, I become immediately conscious of something that presses it so forcibly to the glass, that I cannot release it. The prop is now gone; I have no pressure under my hand; a column of air 45 miles high forces it down by its weight, and I must let in the air under it, before the hand can be withdrawn.

Mr. Coates computed that the weight of the air which presses upon the whole surface of the earth, is equal to that of a globe of lead sixty miles in diameter.

a A column of air of the height of the atmosphere, when greatest, is equal to a column of water 35 feet high, or a column of mercury of the same size 30 inches high. Hence water will not rise in a pump more than 35 feet, nor mercury in a barometer stand higher than 301⁄2 inches. See Additional Notes, No. 12.

A quart measure of atmospheric air weighs about seventeen grains.

If there were no atmosphere surrounding the earth, only that part of the sky would appear light in which the sun was placed; and if a person should turn his back to the sun, he would directly perceive it as dark as night; for in that case there would be no substance to reflect the rays of the sun to kis

of those dews and rains which make the earth

fruitful 2.

Have you learnt any thing of the nature of the air which forms the atmosphere of this earth?

This immense atmosphere is composed of a mixture chiefly of two different airs, which are combined in certain proportions.

eyes. It is owing to refraction that the sun enlightens the earth some time before it rises, and some time after it sets. See this explained with perspicuity by Gregory, in his Astronomical Lessons, page 78-82.

The atmosphere is the cause of evaporation; it is the atmospheric air which holds the aqueous vapours in solution, and preserves them in a gaseous state till they are condensed again into rain.

For the discovery of the composition of atmospheric air we are indebted to Scheele, whose genius when very young enabled him to break the trammels of a dependent situation, and whose subsequent investigations of Nature have immortalized his memory.

Pure atmospheric air is composed of three gaseous substances only, but it is perpetually contaminated by a variety of exhalations from the earth. "The atmosphere is a vast laboratory, in which Nature operates immense analyses, solutions, precipitations, and combinations: it is a grand receiver, in which all the attenuated and volatilized productions of terrestrial bodies are received, mingled, agitated, combined, and separated. Notwithstanding this mixture, of which it seems impossible for us to ascertain the nature, atmospheric air is sensibly the same, with regard to its intimate qualities, whereever we examine it."-FOURCROY.

When the nature of atmospheric air began to be understood, it was generally imagined that it was a mere mixture of oxygen gas and nitrogen gas: but we have now some reason to believe that it is a real chemical compound; that is, that the oxygen and nitrogen form atmospheric air by a chemical union. Mr. Dalton, an ingenious modern chemist, is however still of the first opinion; but the subject must be further investigated before the matter will be absolutely decided.

e

According to Sir Humphry Davy, the air of Europe, Asia, Africa, and America, differs very little in the proportion of its ingredients. Journal Royal Instit. vol. i. page 48. By submitting

What do you call these airs which compose our atmosphere?

They are called oxygen airb and nitrogen air.

Are oxygen and nitrogen the only substances which enter into the composition of the atmosphere? No; atmospheric air contains also one part in every thousand of carbonic acid gas, and several adventitious substances.

to a careful analysis the contents of a glass balloon that had been filled with air at the height of 20,000 feet from the earth, it was found similar in every respect to that taken from the surface. Nicholson's Journal, vol. x. 286.

a

Atmospheric air is a mixture of two distinct and solid substances, viz. oxygen and nitrogen rendered aërial by the expansive power of caloric: it likewise contains a portion of carbonic acid gas.

Oxygen and nitrogen, combined in various proportions, form also no less than three other compounds, viz.

Nitric acid consisting of about 4 parts oxygen to 1 of nitrogen. Nitrous gas, of

Nitrous oxide, of

3 do. to 1.

1 do. to 2

do.

do.

The respirable part of atmospheric air has been called oxygen on account of its acidifying principle: the other part has been termed azote, from its known quality of killing all animals that are obliged to breathe it, when separated from oxygen. The terms are taken from the Greek language. I have in this work adopted nitrogen in preference to azote, because it is the base of nitric acid, and it agrees in termination with oxygen and hydrogen. Carbonic acid gas and hydrogen gas are as incapable of supporting life as nitrogen; therefore there is no reason why one should be called azote more than the other.

It should be remarked, that oxygen denotes the solid base of atmospheric air, and that this requires light as well as caloric in order to convert it into oxygen gas. During combustion, the vital air gives out this light in every direction.

The proportion of carbonic acid gas in atmospheric air wa formerly calculated at one per cent.; but Mr. Dalton has lately demonstrated that it does not amount to more than one part in a thousand. Manchester Mem. N. S. vol. i. 254.

If a pure alkali be exposed to the atmosphere, it will gradually absorb carbonic acid. This is also the case with se