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which would otherwise draw them into actual contact, and retain them in a state of absolute immobility and impenetrability.”—Sir J. F. W. Herschel's Discourse, pp. 321-2.

The ultimate particles of matter are thus demonstrably not in contact-"Since all matter may be made to fill a smaller volume by cooling, it is evident that the particles of matter must have space between them.”—Sir H. Davy.

1. Take the quadrant of altitude and bend it: the molecules which compose that portion of its substance which is nearer to its under surface, are now evidently brought closer to each other; they could not, therefore, have been in absolute contact before.

2. The stone is not in absolute contact with the hand which is flinging it; and the fingers themselves do not fly off with the stone, only because the molecules which compose them are packed so closely, as to be sufficiently attracted to each other to resist the motion communicated to them in common with the stone.

D 'Any visible mass of matter then, as of metal, salt, sulphur, &c., we know to be really a collection of dust or minute atoms by some cause made to cohere or cling together: yet there are no hooks connecting them, nor nails, nor glue; and the connection may be broken a thousand times by processes of nature or art; but is always ready to take place again: the cause being no more destroyed in any case by interruption, than the weight of a thing is destroyed by frequent lifting from the ground. Now the cause we know not; but we call it attraction.

"The phenomena of attraction, and its contrary, repulsion, particularly when occurring between bodies at considerable distances from each other, are as inexplicable as any subjects which the human mind has to contemplate." -Dr. Arnott's Elements of Physics, p. 13.

1. The forces of attraction and repulsion, acting between the ultimate particles of matter, and acting, therefore, only at extremely minute distances, are called molecular forces. Sometimes the words 'attraction of cohesion" are employed to denote the former force : This force is not to be confounded with the attraction of gravitation, which acts universally, i. e., at all distances, and quite independently of the states of bodies.-See Gravity (Index.)

THE AERIFORM, LIQUID, AND SOLID STATES.

F "All bodies consist of an assemblage of material particles, held in equilibrio by a cohesive force, which tends

to unite them, and also by a repulsive force, probably caloric, the principle of heat, which tends to separate them. * * * It is evident that the density of substances will depend upon the ratio which the opposing forces of cohesion and repulsion bear to one another."-Mrs. Somerville's Connex. of the Physical Sciences, pp. 124—5.

1. It is important to distinguish the terms mass and bulk: the former refers to the quantity of matter in a body: the latter to the size of its outline or the extensiveness of its outward surface. Thus, a pound of lead is of equal mass with a pound of cork; but its bulk, owing to the compactness of its molecules, is comparatively inconsiderable.

"When particles of the same kind of matter are at such distances from each other, that the cohesion which retains them is insensible, the repulsive principle remains unbalanced, and the particles have a tendency to fly from one another, as in aeriform fluids."—Ibid.

That department of science which treats of the phenomena presented by aerial fluids, is termed Pneumatics.

H "If the particles approach sufficiently near to produce equilibrium between the attractive and repulsive forces, but not near enough to admit of any influence from their form, perfect mobility will exist among them, resulting from the similarity of their attractions, and they will offer great resistance when compressed: properties which characterize liquids, in which the repulsive principle is greater than in the gases."—Ibid.

That department of science which treats of the equilibrium of liquids, is termed Hydrostatics.

1. At the depth of 1000 fathoms, the water of the sea has its bulk so diminished by the pressure of the water above, as that 99 gallons of it, if brought to the surface, would fill 100 gallons there. But 110 pints of common air, may be driven into the hollow ball, of one pint capacity, attached to an air gun; and a stand has been constructed for the support of a gas-light, into which so much gas might be condensed as would feed the flame for several evenings.

K "When the distances between the particles are still less, solids are formed, in consequence of the preponderating force of cohesion."-Ibid.

1. We may understand from G and H, that the term "liquid" is applied only "to such fluids as do not, though quite at liberty, attempt to dilate themselves beyond a certain point." Some fluids,

PRESSURE APPLIED TO FLUIDS.

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however, which exist even in the liquid form under ordinary circumstances of pressure, &c., become aeriform when that pressure is removed.

2. Ether exists as a liquid at the earth's surface, only because of the pressure of the atmosphere, and cannot be contained in an open vessel on the top of a lofty mountain.

Law of Pressure with respect to Fluids.

We conclude these remarks and selections with one very important as it regards our circumstances :—

Whilst pressure, applied to a solid, is transmitted only in that direction in which it is applied; in all fluids, whether aeriform or liquid, in consequence of the perfect freedom of their parts to move amongst themselves, each of the molecules tends to recede from an excess of pressure on one side, and thus bears upon all the rest, and distributes the pressure amongst its neighbours." Hence the remarkable hydrostatic law of "equal pressure in all direc

tions."

1. Suppose a bottle, of any shape whatever, to be made with twenty mouths, distributed over its surface as the quills of a porcupine; and nineteen of these mouths being corked, let it be exactly filled with water by means of the twentieth: any pressure tending to force a cork into this twentieth mouth, would tend equally to drive out all the others.

2. "The mass of the human body may be considered as an accumulation of solid atoms, each separately immersed in a fluid. This being the case, it follows that the pressure upon any portion of the external surface of the body, is propagated equally throughout its substance, by the intervention of the fluids which permeate it; and that each solid particle thus sustains pressures equal in every possible direction; so that, by reason of these pressures, it can have no tendency to move either in one direction or another."-Moseley's Mechanics, p. 277.

D

THE ATMOSPHERE, OCEAN, ETC.

M The globe we inhabit is encased in a fluid, as a kernel of a nut in its shell: and, of its immediate surface, not less than three-fourths are covered with a liquid. On this account it is highly important to an apprehension of some even of the most simple circumstances in nature, to attend particularly to the phenomena of the fluid state. It is hoped that the illustrations given, as they are necessarily few, may serve as hints for further enquiry.

*

1. "We are living in a fluid at the bottom of an ocean, as we see fish to be living in the sea, receiving large quantities of it at every instant into our bodies, and exhaling it. * Philosophers reasoned and speculated for 2000 years, on the subject of the atmosphere, before they discovered that it was material, a fluid, and had weight."-Moseley's Mechanics, p. 289.

2. Sir J. W. Herschel compares the relative bulk of the atmosphere to the downy skin of a peach when contrasted with the dimensions of the fruit.

3. At the level of the sea, 100 cubic inches of air, (about as much as would be contained in a box having its length, width, and depth, equal to the width of this book,) are found to weigh 31 grains; but the existence of this mass of air in such a bulk, is owing to the compression of the air above it. The density of the atmosphere rapidly decreases as we ascend. At the top of Mont Blanc, (an elevation of three miles,) the traveller has passed through half the mass of this ocean of air, although there are yet more than 40 miles of it above him.

4. The atmosphere has been aptly compared to wisps of wool, which being laid one upon another from the floor to the ceiling of a room, would half of them be contained in that portion of the pile which reached only a very few inches from the floor.

5. It is shown by experiments in pneumatics, that the pressure of the atmosphere at the average surface of the earth, is equal to a force of 15 pounds on every square inch of the surface of any body, whatever its position. Fifteen pounds are equivalent to the weight of the quantity of mercury that fills a tube or bottle of one inch bore, and about thirty inches long; or to the weight of water filling a pipe of that bore and thirty-four feet long. Accordingly, if either of these vessels, having been closed at one end, and so filled, be carefully inverted perpendicularly into a basin of fluid, the pressure of the atmosphere upon the surface of the fluid will be an equipoise to what is contained in the tube, and will support it. This constitutes the instrument called the barometer.*

* The barometer is supported less by half an inch on the top of St. Paul's, than at the level of the Thames. On Mont Blanc, only the half column of mercury, or fifteen inches of it, is supported. In De Lussac's balloon ascent, (note to def. 27,) the mercury fell below

twelve inches.

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NWe may judge, from the above experiments, that the whole body of the atmosphere is equal to a covering of water thirty-four feet deep; or to a covering of quicksilver of the depth of thirty inches.

1. The body of a man of ordinary size, presenting a surface of about 2000 square inches, bears a pressure (reckoning in all directions) of about fourteen tons. The arrangements of Infinite Wisdom have provided for this pressure on the human frame, the juices of the body having just such a tendency to expand as this pressure is fitted to counteract. In climbing the Andes, Baron Humboldt found that the blood burst from his lips and ears.

2. "By that admirable property of the equal distribution of fluid pressure, (L, p. 169,) not only are we enabled to sustain the 30,000 pounds' weight of atmospheric pressure, without feeling it, but that pressure may be doubled by immersing the body thirty-six feet under water in a diving-bell, and yet no single nerve, not even the most delicate of the millions which overspread the body, will, by reason of that pressure, experience the least perceptible excitement."Moseley's Mechanics.

3. It is because the pressure of the atmosphere, like that of every other fluid, is communicated equally in all directions, that the boy's leather sucker carries the weighty stone on which he places it; that the common house fly walks on the ceiling; that the snail can crawl, sustaining the burthen of its shell in any position, &c.

*

P The united masses of the ocean and atmosphere, are but insignificant when contrasted with the quantity of solid matter in the globe. The greatest depth of the sea is estimated at five miles; (a) and if that which is now the "dry land," were levelled and distributed, so as to form its bed, its uniform depth would be very inconsiderable. We have seen (M 5, p. 170) that the mass of the atmosphere, if sufficiently condensed, would be equal in bulk to an ocean of water of about thirty-four feet depth; so that, regarding

Those most skilful in microscopic observations, have great difficulty in detecting the sucker in the fly's foot. The Acarus frequently found in great numbers infesting the common blue beetle, and so much in need of the means of firm adhesion, whilst the beetle "wheels his drony flight," is provided, on each foot, with a beautiful transparent sucker, attached by a fine muscular cord. A common microscope will show this with a very moderate power; and if the little animal be carefully shut in between the two glasses of the "live box," and very slightly compressed, it will exhibit these suckers in action. The author is not aware that this object, exquisitely illustrative as it is, is noticed in any publication on the microscope.

(a) The mean depth of the Pacific, is supposed to be about four miles; that of the Atlantic, about three miles only.