of mechanical principles to the motions of the heavenly bodies. We
trace the progress of astronomy through Hipparchus and Ptolemy; and,
after a long halt, through Copernicus, Galileo, Tycho Brahe, and
Kepler; while from the high table-land of thought occupied by these
men, Newton shoots upwards like a peak, overlooking all others from
his dominant elevation.

But other objects than the motions of the stars attracted the
attention of the ancient world. Light was a familiar phenomenon, and
from the earliest times we find men's minds busy with the attempt to
render some account of it. But without _experiment_, which belongs to
a later stage of scientific development, little progress could be here
made. The ancients, accordingly, were far less successful in dealing
with light than in dealing with solar and stellar motions. Still they
did make some progress. They satisfied themselves that light moved in
straight lines; they knew also that light was reflected from polished
surfaces, and that the angle of incidence was equal to the angle of
reflection. These two results of ancient scientific curiosity
constitute the starting-point of our present course of lectures.

But in the first place it will be useful to say a few words regarding
the source of light to be employed in our experiments. The rusting of
iron is, to all intents and purposes, the slow burning of iron. It
develops heat, and, if the heat be preserved, a high temperature may
be thus attained. The destruction of the first Atlantic cable was
probably due to heat developed in this way. Other metals are still
more combustible than iron. You may ignite strips of zinc in a candle
flame, and cause them to burn almost like strips of paper. But we must
now expand our definition of combustion, and include under this term,
not only combustion in air, but also combustion in liquids. Water, for