Expansion, the property displayed by matter of enlarging in bulk by diminution of pressure, increase of heat, or in a few instances by increase of cold, and also of moisture. It is seen in solids in the common operation of setting the tire of a wheel; the iron ring, being heated in the circle of burning chips and coals, enlarges in bulk so as easily to slip over the felly, which it compresses tightly as it grows cool on the application of cold water. It is seen in liquids in the rise of mercury in the thermometer; and in aeriform bodies in the ascending currents of heated air, or more plainly in the bursting of a tight bladder as the air it encloses swells by exposure to heat. The amount of expansion exhibited by different bodies by any given increase of heat is very various. Those only which exist in the aeriform state, or as vapors, can be classed together in this respect. They all expand very nearly if not exactly alike by the same increase of temperature. Like air they increase in bulk from the freezing to the boiling point, so that, according to Gay-Lus-sac, 100 measures at the lower degree fill 1371/2i at the higher.

For each degree of Fahrenheit the expansion of air, according to the accurate determinations of Regnault, is, under a constant volume, 1/490 of its volume; for the less condensable gases it is perceptibly larger. Each solid body has its own rate of expansion, which however is not uniform for equal increments of temperature, but increases at high degrees in a foster ratio. This, unless special allowance is made for it in the graduation, introduces error in thermometers, those marked off in equal divisions for the high degrees evidently not being correct. Another source of error in these instruments is the unequal expansion of the different materials. The mercury from the freezing to the boiling point of water expands, according to Regnault, in volume 1 part in 55.08; between the latter and 392°, 1 in 54.61; and between this and 572°, 1 in 54.01. Glass expands in the same range of temperature, in the first division, 1/387.5; in the second, 1/328.9; and in the third, 1/286.5 In a mercurial thermometer it is the difference of expansion between the mercury and the glass that is indicated, and the temperature indicated by 586° would correspond to 667° determined by the expansion of glass alone, or to 572° by the air thermometer.

Various instruments called pyrometers have been devised to determine high degrees of temperature by the amount of expansion of bars of different metals. They are all approximate only in their results, unless the rate of expansion of the metal bars has been accurately investigated by the help of the air thermometer; and the labor attending such a study has rarely been bestowed upon these instruments, which in every form are now generally superseded by the air thermometer itself or by the electric pyrometer of Siemens. (See Pyrometer, and Thermometer.) The expansions of various solids from 32° to 212° are presented in the following table:

NAMES.

Expansion in length.

Expansion in bulk.

Authorities.

Zinc, cast.....

1 in 836

1 in 112

Daniell.

" sheet....

1 " 340

1 " 113

Smeaton.

Lead.........

1 " 351

1 " 117

Lavoisier and La-place.

Tin...........

1 " 516

1 " 172

Silver........

1 " 524

1 " 175

Brass

1 " 536

1 " 179

Copper.......

1 " 532

1 " 194

Gold..........

1 " 682

1 " 227

Bismuth......

1 " 712

1 " 239

Smeaton.

Iron..........

1 " 846

1 " 282

Dulong and Petit.

Antimony...

1 " 923

1 " 307

Smeaton.

Untempered steel.........

1 " 926

1 " 309

Lavoisier and Laplace.

Palladium...

1 " 1,000

1 " 333

Wollaston.

Platinum.....

1 " 1,131

1 " 377

Dulong and Petit.

Glass without lead........

1 " 1,148

1 " 382

Flint glass....

1 " 1,248

1 " 416

Lavoisier and Laplace.

The expansion in bulk is found by measurement to be about three times the linear expansion, as it should be on geometrical principles of the relations between the side and the volume of a cube. When metals become liquid by fusion, a change takes place in their density; their specific gravity increases in the cases of iron, bismuth, and antimony, as is shown by solid pieces floating upon the surface of a melted mass of the same metal. Thus it is that in castings the mould is entirely filled in its minutest parts. On the other hand, phosphorus, mercury, gold, silver, copper, and many other substances contract as they become solid; and this is the reason why coins of the last three metals cannot be cast, but require to be stamped.-A great difference is shown in the amount of expansion of different liquids; thus water gains 1/9 in bulk when its temperature is raised from 32° to 312°, oil of turpentine 1/14, and mercury in a glass tube 1/65. A remarkable exception to the general law of expansion of liquids in proportion as they are heated is shown in the case of pure water.

When this is cooled from the temperature of 60° it continues to contract until it reaches 39.2°. From this point it expands until it freezes at 32°, its rate of expansion being about the same from 39° whether it is heated or cooled; but if kept perfectly quiescent, Despretz found that below 32° water retains its liquidity and continues to contract. He gives the following determinations:

Temperature, Centigrade.

Density.

Temperature, Centigrade.

Density.

-9°

0.998,371

+ 3°

0.999,999

6

0.990,082

4

1.000,000

3

0.999,577

5

0.999,999

0

0.999,873

6

0.999,969

An important beneficial effect of this peculiarity in the expansion of water is seen in the protection it affords to the natural bodies of this fluid, as lakes and ponds, against being frozen throughout. For, as the surface of the water is cooled below 39° by the cold air above, this portion by its expansion becomes specifically lighter than the water below, and consequently remains at the top. At 32° a covering of ice forms over the water, which being a poor conductor of heat preserves the great body of water from falling to a lower temperature than 39°, the point of its greatest density. The passage from the liquid to the solid state on the abstraction of heat is determined to a very con-siderable extent by the superficial tension of the liquid; thus Despretz finds that in fine capillary tubes water may be cooled to -20° C. (-4° F.) without solidification.-So great a power is exerted by the contraction of metals on cooling after being expanded by heating, that this has been applied as a mechanical force, as in the bringing together of heavy walls of buildings which had separated by unequal settling.

Strong iron bars are passed horizontally through the opposite walls, and being heated throughout their length are closely keyed up and then allowed to cool; and the process is repeated until the desired effect is obtained. This suggests the danger of inserting bars of metal closely in walls of masonry, as the force exerted by their expansion tends to thrust portions of the wall out of place. The expansion of water has been practically applied to the rending of rocks, the fluid being poured into the fissures and allowed to freeze. This is one of the most efficient agents employed by nature for the disintegration of rocky cliffs. The expansion by access of moisture ^exhibited in the swelling of the fibre of wood or of ropes. This, too, is sometimes employed as a powerful mechanical force, as by inserting wedges of wood into cracks, or into holes drilled for the purpose in rocks, and then covering the wood with water. As this is absorbed, the wood slowly expands, exerting a steady pressure of surprising force.

The presence of moisture in the atmosphere is ascertained by instruments based on this principle. (See Hygrometer.) For the effect of expansion of steam, see Steam.