RAREFACTION, in physics, is the making a body to expand or occupy room or space, without the accession of new matter. It is by rarefaction that gunpowder takes effect ; and to the same pritciple also we owe eulipiles, thermo. mett rs, &c. The degree to which air is rarefiable exceeds all imagination ; per haps indeed its degree of expansion is absolutely beyond all limits. Upon the rarefaction of tie air is fbunded the me thod of measuring altitudes by the baro meter ; in all cases of which the rarity of the air is found to be inversely as the force that compresses it, or inversely as the weight of all the air above it at any place.
The open air, in which we breathe, says Sir Isaac Newton, is 8 or 900 times lighter than water, and by consequence 8 or 900 times rarer. And since the air is compressed by the weight of the in cumbent atmosphere, and the density of the air is proportionable to the compress ing force, it follows by computation, that at the height of about seven Engligh miles from the earth, the air is four times rarer than at the surface of the earth ; and at the height of 14 miles, it is 16 times rarer than at the surface of the earth ; and at the height of 21, 28, or 35 miles, it is res pectively 64, 256, or 1024 times rarer, or thereabouts ; and at the height of 70, 140, and 210 miles, it is about 1,000,000, 1,000,000,000,000, or 1,000,000,000,003, 000,&c.
Mr. Cotes has found, from experiments made with a thermometer, that linseed 'oil is rarefied in the proportion of 40 to 39 in the heat of the human body ; in that of 15 to 14, in that degree of heat wherein water is made to boil; in the proportion of 15 to 13, in that degree of heat wherein melted tin begins to har den ; and finally, in the proportion of 23 to 20, in that degree wherein melted tin arrives at a perfect solidity. The same author discovered, that the rarefaction of the air, in the same degree of heat, is ten times greater than that of the linseed-oil ; and the rarefaction of the oil, about fif teen times greater. than that of the spirit of wine.