. They
are now parallel, and therefore to a certain extent diffused. We place
a convex lens (L) in the path of the beam; the light is converged to a
focus (C), and at that focus paper is not only pierced, but it is
instantly set ablaze.

Many metals may be burned up in the same way. In our first lecture
the combustibility of zinc was mentioned. Placing a strip of
sheet-zinc at this focus, it is instantly ignited, burning with its
characteristic purple flame. And now I will substitute for our glass
lens (L) one of a more novel character. In a smooth iron mould a lens
of pellucid ice has been formed. Placing it in the position occupied a
moment ago by the glass lens, I can see the beam brought to a sharp
focus. At the focus I place, a bit of black paper, with a little
gun-cotton folded up within it. The paper immediately ignites and the
cotton explodes. Strange, is it not, that the beam should possess such
heating power after having passed through so cold a substance? In his
arctic expeditions Dr. Scoresby succeeded in exploding gunpowder by
the sun's rays, converged by large lenses of ice; here we have
succeeded in producing the effect with a small lens, and with a
terrestrial source of heat.

In this experiment, you observe that, before the beam reaches the
ice-lens, it has passed through a glass cell containing water. The
beam is thus sifted of constituents, which, if permitted to fall upon
the lens, would injure its surface, and blur the focus. And this leads
me to say an anticipatory word regarding transparency. In our first
lecture we entered fully into the production of colours by absorption,
and we spoke repeatedly of the quenching of the rays of light. Did
this mean that the light was altogether annihilated? By no means. It
was simply so lowered in refrangibility as to escape the visual range.
It was converted into heat. Our red ribbon in the green of the
spectrum quenched the green, but if suitably examined its temperature
would have been found raised. Our green ribbon in the red of the
spectrum quenched the red, but its temperature at the same time was
augmented to a degree exactly equivalent to the light extinguished.
Our black ribbon, when passed through the spectrum, was found
competent to quench all its colours; but at every stage of its
progress an amount of heat was generated in the ribbon exactly
equivalent to the light lost. It is only when _absorption_ takes place
that heat is thus produced: and hea