EXPANSION, in physics, is the increase in the bulk of bodies, in consequence of a rise in their temperature. This is one of the most general effects of heat, being com mon to all forms of matter, whether solid, liquid or gaseous. Expansion of three kinds is recognized: (1) linear (2) super ficial ; and (3) cubical. The last only is applicable to liquids and gases, as they have no definite form. Most solids and liquids ex pand uniformly as their temperature increases, each substance having its own peculiar rate of expansion. This statement is true, however, only for usual temperatures. It has been found that as temperatures are increased beyond nor mal limits the rate of expansion also increases. The ratio of increase in bulk for each degree rise in temperature as referred to the original volume at a temperature of zero is called the coefficient of expansion. Solids expand equally in all directions, so that the coefficient of cubical expansion for solids is three times the linear co efficient. Liquids expand much more for the same rise in temperature than do solids, and gases still more proportionally. With gases, moreover, the bulk depends very largely upon the pressure acting upon it, gases being readily compressible. However, gases do not differ ma terially in their coefficients of expansion. It is found that under a constant pressure the co-efficient of cubical expansion for all gases is about 1/490. The expansion of liquids varies considerably, but, in general, the denser the fluid the less the expan sion; thus water expands more than mer cury, and alcohol more than water; and, com monly, the greater the heat the greater the ex pansion; but this is not universal, for there are cases in which expansion is produced, not by an increase, but by a diminution of temperature. Water furnishes us with the most remarkable instance of this kind. Its maximum of density corresponds with 39.2° F. This fact is of the utmost importance in the economy of nature. When the surface of rivers and lakes is cooled the upper or surface layer of water sinks and warmer water from below takes its place till the whole mass is cooled to 40°. After this the circulation ceases; the colder layer being less dense remains at the top until it freezes. The maximum density point of sea water is consider ably lower than that of fresh water, and varies with the quantities of the salt contained in it.
The expansion of water is about the same for any number of degrees above or below the max imum density point. Thus, if we heat water 5° above 39.2° it occupies the same bulk as it does when cooled down to below The force with which water ex pands in the act of freezing is shown when glass bottles are filled with water and sealed; the glass is broken in pieces when the water freezes. A brass globe, whose cavity is an inch in diameter, may be burst by filling it with water and freezing it ; and the force necessary for this effect is 27,720 pounds. The expansive force of freezing water is due to the tendency which water in solidifying is observed to have to arrange its particles so as to form prismatic crystals, crossing each other at angles of 60 degrees and 120 degrees. Various methods have been tried to ascertain the specific gravity of ice at ; that which succeeded best was to dilute alcohol with water till a mass of solid ice put into it remained in any part of the liquid with out either sinking or rising. The specific gravity of such a liquid is 0.92, which, of course, is the specific gravity of ice, supposing the specific gravity of water at 60° to be 1. This is an expansion much greater than water experiences even when heated to its boiling-point. We see from this, that water at the instant of solidification rceives a sudden and considerable augmentation of bulk. See HEAT.
The coefficients of expansion of some of the commoner solids and liquids on the Fahrenheit scale are approximately as follows: Glass, 1/75,000; iron, 1/50,000; gold, 1/40,000; copper, 1/36,000; silver and brass, 1/33,000; tin, 1/31,000; lead and zinc, 1/23,000; mercury, 1/11,700; water, 1/3,870; ether, 1/2,570; chlo roform, 1/1,150.
Economically, the importance of expansion is very large, particularly in the temperate zone. Here the variation of natural temperatures be tween winter and summer •is about 80°. In all buildings, bridges, concrete constructions, etc., expansion joints are inserted to take up the variation due to the changes in temperature. In instruments of precision this variation be comes of the utmost importance, requiring deli cate adjustments, as in the compensating pen dulum. See PENDULUM.