15. If a body absorbs heat, its volume will be changed correspondingly. Nearly all bodies expand when heated; a few substances are known which contract, but these exceptions are of no practical importance.

Air and all other gases expand uniformly for each degree of rise in temperature above zero. Air, at zero F., will expand 1/460 of its volume for each degree of rise in temperature. Thus, air at 70° will have a volume equal to 1 +70/460, or 530/460 of its volume at zero, if its tension remains unchanged. By tension is meant the pressure which a gas exerts on the vessel which confines it.

The volume of water does not increase at the same rate as the temperature; therefore, the increased volume due to a rise of temperature can only be determined by experiment.

Water is practically incompressible, and it expands with as much force as ordinary metals. When water is heated or cooled, the vessel or pipes which contain it expand or contract also, but to a less degree.

The expansion of wood by heat is so small that it may be neglected.

16. A consideration of the nature of heat readily explains the phenomenon of expansion. When a body, as, for example, a bar of iron, is heated, its molecules begin to move more rapidly and in longer paths. The more the body is heated and the higher its temperature, the farther apart the molecules are driven, and consequently the greater the volume of the body.

The linear expansion, or extension, of metals for one degree rise of temperature is given in the following table:

Table 4


Increase of

Length in 1 Foot for an Increase in Temperature of 1° F.


Cast iron....


Wrought iron.


Steel tubes. . .









Increase of

Length in 1 Foot for an Increase in Temperature of 1° F.




Tin .....






Firebrick .....




17. The amount of the expansion or contraction of a bar or pipe, having a given length, which will be caused by any given change in its temperature, may be found as follows:

Rule 1

Multiply the length in feet by the number of degrees of change in temperature. Multiply this product by the coefficient given in the above table, for the material employed. The result will be the change of length in inches.


How much will a steel tube 14 feet long expand, if its temperature is raised 80°?


From Table 4, the coefficient of linear expansion per unit of length for a rise in temperature of 1° is found to be .0000719 for steel tubes. Hence, using rule 1, the increase of length is 14 X 80 X .0000719 = .0805 in. Ans.

18. If metallic bodies are heated above a certain degree (which varies for different metals), and the heat is continued for any considerable length of time, the metal will become permanently elongated, and upon cooling will not contract to its original dimensions. The metal is then said to be swelled. Thus, grate bars in a furnace, or pipes which are exposed to intense heat, will increase considerably in length during long use. The strength of the metal deteriorates at the same time. Thus, plates or parts of furnaces which arc-unduly heated will swell permanently, and bulge or crack the adjoining parts.