We will not now go back to man’s first intellectual gropings; much less shall we enter upon the thorny discussion as to how the groping man arose.  We will take him at that stage of his development, when he became possessed of the apparatus of thought and the power of using it.  For a time—­and that historically a long one—­he was limited to mere observation, accepting what Nature offered, and confining intellectual action to it alone.  The apparent motions of sun and stars first drew towards them the questionings of the intellect, and accordingly astronomy was the first science developed.  Slowly, and with difficulty, the notion of natural forces took root in the human mind.  Slowly, and with difficulty, the science of mechanics had to grow out of this notion; and slowly at last came the full application 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 example, contains a store of oxygen, which may unite with, and consume, a metal immersed in it; it is from this kind of combustion that we are to derive the heat and light employed in our present course.