in Energy \ tJie Power of Nature 41 



have small capacity. It is level alone that decides the 

 direction in which the liquid will flow when two vessels are 

 connected by a pipe. Similarly there are some kinds of 

 matter one pound of which requires a great deal of heat to 

 raise its temperature by one degree, while an equal mass of 

 others is raised in temperature to the same amount by very 

 little heat. The former class of substances are said to have 

 a great capacity for heat, or, as it is sometimes called, a high 

 specific heat. Thirty times as much heat is required to 

 raise the temperature of i Ib. of water i as to raise the 

 temperature of the same mass of mercury by the same 

 amount. Water, indeed, has the greatest capacity for heat 

 of any substance known. On the same fire, if other con- 

 ditions are the same, mercury becomes as hot in a minute 

 as an equal mass of water does in half an hour ; but then as 

 a necessary consequence heated mercury cools as much in 

 a minute as an equal and equally heated mass of water does 

 in half an hour. 



67. Expansion by Heat. When the temperature of 

 matter is raised the oscillations of the particles are not only 

 more rapid but of greater amplitude. Each particle occupies 

 a greater space in its longer swing, and consequently the 

 volume occupied by the matter is increased and the density 

 diminished. Expansion of volume by heat takes place in 

 solids, liquids, and gases alike, though its amount is different 

 in each kind of matter and is always greater for gases and 

 liquids than for solids. The lengthening of a bar of iron 

 when heated or its contraction when cooled takes place with 

 nearly irresistible force. The rails of the railway 400 miles 

 long between London and Edinburgh are nearly 1000 

 feet longer on a summer afternoon than on a winter 

 night. The expansion of a metal rod is often used as a 

 measure of temperature ; but thermometers (see 440) are 

 usually constructed by taking advantage of the greater 

 expansion of liquids or gases. If heat is applied to the 

 lower part of a vessel containing liquid the layer next the 

 source of heat is raised in temperature, expands, and becom- 

 ing less dense rises to the surface, allowing the denser 

 liquid above to subside to the bottom and get heated in its 



