bodies at the same temperature, just as by specific gravity I understand
the relative quantities of matter in bodies of the same bulk. Equal weights
of quicksilver and water at the same temperature do not contain the same
quantities of heat, any more than equal bulks of those liquids contain the
same quantity of matter. The absolute quantity of heat in any body is not
known; but the relative heat of bodies at the same temperature, or in other
words their specific heats, have been ascertained and arranged in tables,--
the specific heat of water being taken as unity.

138. _Q._--In what way does the specific heat of a body enable the quantity
of heat in it to be determined?

_A._--If any body has only half the specific heat of water, then a pound of
that body will, at any given temperature, have only half the heat in it
that is in a pound of water at the same temperature. The specific heat of
air is .2669, that of water being 1; or it is 3.75 times less than that of
water. An amount of heat, therefore, which would raise a pound of water 1
degree would raise a pound of air 3.75 degrees.



COMBUSTION.

139. _Q._--What is the nature of combustion?

_A._--Combustion is nothing more than an energetic chemical combination,
or, in other words, it is the mutual neutralization of opposing
electricities. When coal is brought to a high temperature it acquires a
strong affinity for oxygen, and combination with oxygen will produce more
than sufficient heat to maintain the original temperature; so that part of