Take an electric lamp, giving a strong beam of light and heat, and with a plano-convex lens gather it into a single beam and direct it upon a thermometer, twenty feet away, that is made of glass and filled with air.  The [Page 32] expansion or contraction of this air will indicate the varying amounts of heat.  Watch your air-thermometer, on which the beam of heat is pouring, for the result.  There is none.  And yet there is a strong current of heat there.  Put another kind of test of heat beyond it and it appears; coat the air-thermometer with a bit of black cloth, and that will absorb heat and reveal it.  But why not at first?  Because the glass lens stops all the heat that can affect glass.  The twenty feet of air absorbs all the heat that affects air, and no kind of heat is left to affect an instrument made of glass and air; but there are kinds of heat enough to affect instruments made of other things.

A very strong current of heat may be sent right through the heart of a block of ice without melting the ice at all or cooling off the heat in the least.  It is done in this way:  Send the beam of heat through water in a glass trough, and this absorbs all the heat that can affect water or ice, getting itself hot, and leaving all other kinds of heat to go through the ice beyond; and appropriate tests show that as much heat comes out on the other side as goes in on this side, and it does not melt the ice at all.  Gunpowder may be exploded by heat sent through ice.  Dr. Kane, years ago, made this experiment.  He was coming down from the north, and fell in with some Esquimaux, whom he was anxious to conciliate.  He said to the old wizard of the tribe, “I am a wizard; I can bring the sun down out of the heavens with a piece of ice.”  That was a good, deal to say in a country where there was so little sun.  “So,” he writes, “I took my hatchet, chipped a small piece of ice into the form of a double-convex lens, [Page 33] smoothed it with my warm hands, held it up to the sun, and, as the old man was blind, I kindly burned a blister on the back of his hand to show him I could do it.”

These are simple illustrations of the various kinds of heat.  The best furnace or stove ever invented consumes fifteen times as much fuel to produce a given amount of heat as the furnace in our bodies consumes to produce a similar amount.  We lay in our supplies of carbon at the breakfast, dinner, and supper table, and keep ourselves warm by economically burning it with the oxygen we breathe.

Heat associated with light has very different qualities from that which is not.  Sunlight melts ice in the middle, bottom, and top at once.  Ice in the spring-time is honey-combed throughout.  A piece of ice set in the summer sunshine crumbles into separate crystals.  Dark heat only melts the surface.

Nearly all the heat of the sun passes through glass without hinderance; but take heat from white-hot platinum and only seventy-six per cent. of it goes through glass, twenty-four per cent. being so constituted that it cannot pass with facility.  Of heat from copper at 752 deg. only six per cent. can go through glass, the other ninety-four per cent. being absorbed by it.