RAREFACTION is an augmentation of the intervals between the particles of aeriform fluids, so that the same number of particles are made to occupy a volume greater than that under which they were previously contained. The term is used in opposition to condensa tion, and in the same sense as dilatation, which last is applied both to fluids and solids. Rarefaction or dilatation is caused by a repulsive power existing either in the particles of bodies or in those of the caloric between them, by which power the particles are made to recede from one another when not prevented by some external resist ance. [ATTRACT/ON.
The experiments of Lavoisier and Laplace have shown that, between the temperatures of freezing and of boiling water, the dilatations of all metals and of the fluids called non-elastic are constantly propor tional to the increments of temperature ; but beyond the temperature of boiling water the experiments of Dulong and Petit indicate that the dilatations increase in a higher ratio. A remarkable circumstance is observed in the state of water when near congelation. On being cooled to a temperature of about 38'8 of Fahr., its volume remains stationary through a small range, and in this state water seems to have attained its maximum of density ; for, on continuing the cooling pro cess, the water begins to expand, and it continues to do eo until it is converted into ice. In fact, just before water freezes it occupies a space as great as it did at 48°. If water at 39°.2 be taken as 1, at 32° its density= From the experiments of M. Gay-Lussac and Dr. Dalton it has been ascertained that, under equal external pressures, the rarefactions of all dry gases and of the aeriform substances produced by the evapo ration of liquids are equal at equal temperatures, between the points of freezing and boiling water, and that they vary in volume propor tionally to the increments of heat expressed by the expansions of mercury in the thermometer. [AIR; HEAT; GAS; PNEumertos.]1 The density or closeness of the aerial particles in any given portion of the atmosphere depends on the pressure or weight of the column of air above the given point ; and by the law of Boyle or Mariotte, whatever be the temperature, provided it be constant, the density is proportional to the pressure; or, conversely, the rarefaction is inversely proportional to the pressure. It has been proved IP:mu
stnuce] that the densities of the strata of air decrease upwards in a geometrical progression when the altitudes of the strata increase by equal increments ; and this is equivalent to saying that the rare factions of the air at such altitudes increase in a geometrical progres sion. Now if e be any point on the surface of the earth, and c any point above it, the formula n o (in fathoms)=10670 x log. density at A density at C [Pxrumarics], will afford the means of computing the:rarefaction of the air at any point c, when its height above a is given. But if the density ate be considered as unity (the temperature being= the equation may be transformed into In which, substituting for A C any given value in fathoms, the rare faction may be found. For example, let A c=3212 fathoms (3'65 milts); then the second member of the equation will become '30103 (=log. 2), which shows that at the height of about 3] miles the rare faction of the atmosphere is twice as great as at tho surface of the earth ; and by forming the progression of heights 0 , 3'65 , 7'3 , 3:c. in miles, wo have for the corresponding rarefactions 1 , 2 , 4 , 8, &c.
For the rarefaction of the air in an air-pump, after any number of strokes of the piston, see AIR-PUMP.
The limits to which rarefaction may be carried are unknown, but the experiments of Mr. Boyle and others have proved that, by simply removing the external pressure, air may be so ratified that a given volume of the same density as at the surface of the earth will ocenpy a volume more than 13,000 times as great. It has been however dis covered that at very high degrees of rarefaction the elasticity of the air decreases in a higher ratio than the density ; and this may serve to prove that rarefaction cannot take place to an infinite extent. [ATMOSPHERE.]