L
body may seem to be cold or warm according to the condition
in which we have been before.
§ 120. The degree of warming is called temperature,
which is not a quantity of the substance of heat (first theory),
but a quantity of the living power, produced by the vibrating
atoms of a body (second theory). According to this second
theory, which is now accepted in general, heat like sound is
caused by a vibratory motion. 258.
§ 121. 1) There are different sources of heat. They
are: 1) mechanical sources, such as friction, percussion and
pressure; 2) physical sources, that is: solar radiation, terrestrial
heat, changes of condition and electricity; 3) chemical sources,
i.e. chemical combinations, especially combustion. 258.
§ 122. a) Heat due to friction. Flint and steel or two
pebbles rubbed together. 259. The axles of a carriage. 260.
Sliding down a rope. 262. Rubbing of Lucifer matches and safety
matches. 261. Sir H. Davy even succeeded in melting two
pieces of ice by rubbing them together in a vacuum below zero.
Heat is produced in the tools used for sawing, boring, filing etc.
or by rubbing the hands together. 268. Some think, that even
shooting-stars are planetary bodies, which, attracted by the
earth, are raised to incandescence in consequence of their fric
tion with the atmosphere and by the compression of the air.
§ 123. b) Heat due to percussion. A horse trotting
over a hard pavement; iron hammered on an anvil may be rai
sed to red heat.
§ 124. c) Heat due to chemical combination. Chemical
combinations are always accompanied by an elevation of tem
perature: limeslaking; water and sulphuric acid; animal
heat (see § 174); combustion as a combination of the Hydrogen
and Carbon contained in the burning substance with the Oxygen
