Page:The American Cyclopædia (1879) Volume VII.djvu/41

 EXPANSION 33 troduces error in thermometers, those marked off in equal divisions for the high degrees evi- dently not being correct. Another source of error in these instruments is the unequal ex- pansion of the different materials. The mer- cury from the freezing to the boiling point of water expands, according to Regnault, in vol- ume 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, ^. ; in the second, yyW 5 and in tlie. tm ' rd ? yyVy. In. a mercuruu thermometer it is the difference ' of expansion between the mercury and the glass that is indicated, and the tem- perature 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 tem- perature by the amount of expansion of bars of different metals. They are all approximate only in their results, unless the rate of expan- sion of the metal bars has been accurately in- vestigated 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the labor attending such a study has rarely been bestowed upon these instruments, which in every form are now generally superseded by the air thermometer itself or by the electric pyrometer of Siemens.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 PYEOMETEK, and THERMOMETER.) The expansions of various solids from 32 to 212 are presented in the following table : 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 Mgs the mould is entirely filled in its mi- st 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 Temperature, Centigrade. ^9 6 3

Density. Temperature, Centigrade. Density. 0-998,371 0-999,082 0-999,577 0-999,873 + 3 4 5 6 0-999,999 1-000,000 0-999,999 0-999,969 NAMES. Expansion in length. Expansion In bulk. Authorities. Zinc, cast... " sheet.. . Lead Tin 1 la 336 1 840 1 851 1 616 1 524 1 536 1 582 1 682 1 712 1 846 1 923 1 926 1 1,000 1 1,131 1 1,148 1 1,248 1 in 1 || 1 1 1 " 112 113 117 172 175 179 194 227 239 282 307 309 333 377 882 416 Daniell. Smeaton. Lavoisier and La- place. meaton. Dulong and Petit. Smeaton. Lavoisier and Laplace. Wollaston. ( Dulong and Petit. Lavoisier and Laplace. Silver Brass Copper Gold. Bismuth Iron Antimony . . . Untempered steel Palladium Platinum Glass without lead Flint glass 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 $ in bulk when its temperature is raised from 32 to 312, oil of turpentine T V, and mercury in a glass tube -fa. A remarka- ble 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 con- tinues 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 con- tinues to expand. He gives the following de- terminations : An important beneficial effect of this peculiar- ity in the expansion of water is seen in the pro- tection 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 con- ductor of heat preserves the great body of water from falling to a lower temperature than 39, the point of its greatest density. The pas- sage 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 ca- pillary tubes water may be cooled to 20 0. ( 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 un- equal settling. Strong iron bars are passed horizontally through the opposite walls, and being heated throughout their length are close- ly keyed up and then allowed to cool ; and the process is repeated until the desired effect is obtained. This suggests the danger of insert- ing 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 is exhib- ited in the swelling of the fibre of wood or of