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zinc or bismuth; silver with tin, quicksilver, zinc or bismuth; copper with tin, iron, or zinc; lead with zinc or bismuth; or wax with ox tallow; the bulk of the compound is less than what it ought to be, by adding the bulk of one to that of the other.

On the other hand, the pores of some bodies are enlarged by the mixture. Thus, iron mixed with lead, tin, regulus of antimony, zinc, or bismuth; and mercury with tin, lead, zinc, or bismuth, are increased in bulk in a greater proportion, than what should arise from the addition of their bulks..

A Table of specific gravities of different bodies; the thermometer at 50°, and barometer at 29.5 inches.

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ELASTIC FLUIDS.

HITHERTO, we have explained the nature and properties of incompressible and unelastic fluids, in a state of rest. But there are other fluids, which are elastic and compressible, such as the air, in which we breathe; and water, when heated to such a degree, as is sufficient to convert it into vapour, which is then found to be extremely elastic.

We must endeavour to explain the general properties of this kind of fluids, before we shall be able to understand the laws of their motions, or of bodies moving through them. As we always find a sufficient quantity of air in an elastic state, the branch of natural philosophy, which explains the nature and properties of air, is called pneumatics, as being the principal elastic fluid, with which we are acquainted. It may not, however, be improperly considered under the head of hydrostatics; especially when we reflect that it, as well as water, is sometimes found in a fixed, as well as in an elastic state. Whether ever it be found in that intermediate state, in which it can be no farther compressed, we cannot say with absolute certainty. Yet we know, that

it

may be so far compressed in a syringe closed at the end, that no force, with which we are yet acquainted, can urge the piston down to the bottom of the syringe.

It is often found in a fixed and unelastic state in the composition of natural bodies; as is evident from the great quantities that are disengaged by distillation, fermentation, putrefaction, explosions of gunpowder, and chemical solutions and mixtures. Half a drachm of the oil of carroway seed, poured upon a drachm of the compound spirit of nitre, produces such a quantity of air, as is sufficient to blow up an exhausted receiver,

with a force equal to 400 pounds. A strong acid poured upon chalk, disengages a great quantity of air, which before remained fixed and unelastic in the chalk. A cubic inch of pease, by distillation, yielded 396 inches of elastic air; of coarse sugar 126; of Newcastle coal 360 inches; and other substances yielded other quantities.*

That the air is unelastic in its fixed state, appears from this, that a cubic inch of oak by distillation produced 216 inches of elastic air, which is one fourth of the weight of the oak; and this quantity of air could not be compressed into the space of one inch by a force less than 19440 pounds; and would require a still greater force to reduce it to the size of one quarter of an inch. Now this force exerted upon every cubic inch in a tree, is more than sufficient to rend it in pieces. The air, therefore, in a tree, must be in an unelastic

state.

By what secret operation of nature the air is thus deprived of its elasticity, we have not yet sufficient experiments to determine. We know that some bodies absorb it in very considerable quantities. A brimstone. match has been found to absorb one fourth of the air in which it burned, while a tallow candle consumed but one eleventh part, and a rat, one thirteenth part of the air in which it breathed. Thus the operation of heat seems to fix the air in some bodies, while the heat of fermentation disengages it from others.† While the

* These, and all other factitious and permanently elastic airs, are, by modern chemists, termed gases; and are considered as constituent parts of the bodies from which they are extracted.

† Atmospheric air is now well known to consist of two gases, in modern chemistry termed oxygen gas and nitrogen gas, or azote, with a very small portion of a third termed carbonic acid

clastic vapour of water may again be reduced to an incompressible state by a small degree of cold, and by a still greater degree of cold, may be congealed into ice; air seems incongealable by any degree of cold whatsoever. Although air may be much affected with heat and cold, as we shall see presently, yet we know of no degree of cold, that can fix it into a solid body; and in this, it differs from every other elastic fluid, that we know, which, by a proper degree of cold, may be reduced to the incompressible fluid, from whence it was produced, or even to a fixed and permanent state.

After having made these observations, we shall proceed to consider the properties of an elastic fluid, which agree with, or distinguish it from, any other fluid, and which are such as these.

1. The particles of an elastic fluid, when compressed equally in every direction, cannot be at rest, unless they be all at equal distances from each other.

If the particles be nearer to each other in any one part of the fluid, than in another, they will be repelled most on that side, where the distances between them are the least; and consequently the particles which are farthest apart will move closer together, until they are all placed at equal distances from each other, and are equally pressed on every side, when there can be no more motion among them. This will make the density of the fluid the same in every part, as there will not be a greater number of particles squeezed together in any one part of the fluid, into a given space, than in any other part of the fluid.

gus, all intimately united together. The burning of a candle, or any other inflammable body, or the respiration of an animal absorbs the oxygen gas, and renders the residuum unfit either for the support of flame or of animal life.

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