Because its chief effects are to expand, fuse, evaporate, or decompose the substances upon which it acts.
76. What is an attractive agent, in contradistinction to a repulsive agent?
Chemical attraction, or affinity, is an attractive agent—as when bodies seek of their own natures to unite and form some new body.
77. When is a body said to be hot?
When it holds so much caloric that it diffuses heat to surrounding objects.
78. When is a body said to be cold?
When it holds less caloric than surrounding objects, and absorbs heat from them.
79. How may caloric be excited to develop heat?
By any means which cause agitation, or produce an active change in the condition of bodies. Thus friction, percussion, sudden condensation or expansion, chemical combination, and electrical discharges, all develope heat.
80. Why do "burning glasses" appear to set fire to combustible substances?