Examine the water, then, in front of the lamp after the beam has passed through it:  it is sensibly warm, and, if permitted to remain there long enough, it might be made to boil.  This is due to the absorption, by the water, of a certain portion of the electric beam.  But a portion passes through unabsorbed, and does not at all contribute to the heating of the water.  Now, ice is also in great part transparent to these latter rays, and therefore is but little melted by them.  Hence, by employing the portion of the beam transmitted by water, we are able to keep our lens intact, and to produce by means of it a sharply defined focus.  Placed at that focus, white paper is not ignited, because it fails to absorb the rays emergent from the ice-lens.  At the same place, however, black paper instantly burns, because it absorbs the transmitted light.

And here it may be useful to refer to an estimate by Newton, based upon doubtful data, but repeated by various astronomers of eminence since his time.  The comet of 1680, when nearest to the sun, was only a sixth of the sun’s diameter from his surface.  Newton estimated its temperature, in this position, to be more than two thousand times that of molted iron.  Now it is clear from the foregoing experiments that the temperature of the comet could not be inferred from its nearness to the sun.  If its power of absorption were sufficiently low, the comet might carry into the sun’s neighbourhood the chill of stellar space.

Sec. 4. Combustion of a Diamond by Radiant Heat.

The experiment of burning a diamond in oxygen by the concentrated rays of the sun was repeated at Florence, in presence of Sir Humphry Davy, on Tuesday, the 27th of March, 1814.  It is thus described by Faraday:—­’To-day we made the grand experiment of burning the diamond, and certainly the phenomena presented were extremely beautiful and interesting.  A glass globe containing about 22 cubical inches was exhausted of air, and filled with pure oxygen.  The diamond was supported in the centre of this globe.  The Duke’s burning-glass was the instrument used to apply heat to the diamond.  It consists of two double convex lenses, distant from each other about 31/2 feet; the large lens is about 14 or 15 inches in diameter, the smaller one about 3 inches in diameter.  By means of the second lens the focus is very much reduced, and the heat, when the sun shines brightly, rendered very intense.  The diamond was placed in the focus and anxiously watched.  On a sudden Sir H. Davy observed the diamond to burn visibly, and when removed from the focus it was found to be in a state of active and rapid combustion.’