It is owing to this, that rough and
black vessels boil water sooner than smooth and light-colored ones.
Another principle is _Reflection_, by which heat radiated to a
surface is turned back from it when not absorbed or allowed to pass
through; just as a ball rebounds from a wall; just as sound is thrown
back from a hill, making echo; just as rays of light are reflected
from a mirror. And, as with light, the rays of heat are always reflected
from a surface in an angle exactly corresponding to the direction in
which it strikes that surface. Thus, if heated are comes to an object
perpendicularly--that is, at right angles, it will be reflected back
in the same line. If it strikes obliquely, it is reflected obliquely,
at an angle with the surface precisely the same as the angle with which
it first struck. And, of course, if it moves toward the surface and
comes upon it in a line having so small an angle with it as to be
almost parallel with it, the heated air is spread wide and diffused
through a larger space than when the angles are greater and the width
of reflection less.
[Illustration: Fig. 31.]
[Illustration: Fig.
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