COEFFICIENTS OF LINEAR EXPANSION OF SOLIDS.
Aluminium 0.0000222 Pine cLorill34 Gold 0.0000147 Beech 0.101'126 Wroughtiron.... 0.0000121 Ash 0.0000095 Castillo!' 0.0000106 Mahogany.... 0.0000036 Lead. 0.0000271 Vulcanite 0.0000636 Platinum 0.0000093 Paraffin 0.0001303 Copper Quarts, vitri Silver 0.0000297 0.0000192 Ice 0.0000510 Steel 0.0000123 Glass tubing.. 0.0000083 German 0.0000183 (flan. Jena Inver 0.00000087 therm. 59 III 0.0000058 Porcelain 0.0000041 Two notable cases may be remarked. It is seen from the table that the coefficient for glass is very 'close to that for platinum. This fact is taken advantage of in the construction of incandescent electric lamps, and of those scien tific instruments where it is necessary to have a wire pass through glass and leave an air-tight joint. In making the joint, the glass around the hole is softened by heat until it gathers closely around the hot platinum wire. In tool ing, if the coefficient for platinum were higher than that for glass, the platinum would contract more rapidly than the glass and leave a leaky joint. The second case to be noted is that of Guillaume's nickel steel, known as invar. The coefficient of expansion of this metal is so ex tremely small that it is eminently suited to the construction of clock pendulum rods, of survey ing instruments and of standard scales of length, and to many' other purposes in which much expansion now proves an annoyance. Un fortunately the high cost of nickel will pre clude the employment of this wonderful alloy in some cases.
The influence of expansion is seen in rail road tracks. On a cold day 60-foot rails may contract so as to draw apart of an inch. The cables of the Brooklyn bridge sup port the slightly arched roadway. When they sag down in hot weather through. expansion, they tend to make the roadway buckle. This tendency is increased by the expansion of the roadway itself. However, both tendencies were
overcome through the foresight of the engi neers, who provided a telescoping joint in the roadway at the middle of the span. The parts of this joint play in and out about a foot. On hot days clock pendulums grow longer, and so the clocks lose time. Glass when suddenly and hence unevenly heated expands more at one point than at another, thus introducing in ternal strains that cause fracture, but vessels made of vitrified quartz, on account of their extremely low expansibility, resist this tendency to crack; they will endure without injury the sudden application of a blowpipe flame.
In liquids the molecules are so far freed from cohesion that they are able to roll around one another and to wander from one position to another. The small remaining cohesion is assisted by the pressure of the atmosphere or by any other pressure to which the liquid may be subjected and so the molecules in the body of the liquid are prevented from flying directly apart. It is on account of this small resistance to expansion that we find liquids very much more expansible than solids. The term coeffi cient of cubical .expansion is employed to ex press the degree of expansibility of a liquid. It means the fraction of its volume that a liquid expands when its temperature is raised 1° C. The cubical coefficient of a substance is three times as great as its linear coeffi cient, because we measure the effect of ex pansion in length, breadth and thickness, instead of merely noting• the expansion in length. Of course a liquid confined in a tube of unchanging dimension could only expand in length, but the effect in this one direction would be three times as much as it would be if the liquid were allowed to expand proportionally in all three dimensions.