[Illustration:  Fig. 48.]

We will now adopt stronger measures with the radiation.  In this larger camera of blackened tin is placed a lamp, in all particulars similar to those already employed.  But instead of gathering up the rays from the carbon-points by a condensing lens, we gather them up by a concave mirror (m m’, fig. 48), silvered in front and placed behind the carbons (P).  By this mirror we can cause the rays to issue through the orifice in front of the camera, either parallel or convergent.  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 heat is always a result of absorption.