HEAT
385
fcion two images of the jar, formed by rays at right angles
with those direct from the jar to the spectators. The great
advantage of this method is, that the spectators only need
to be about the same height as the jar, to receive perfectly
polarised light all round; and the complementary effect
can always be seen at the same moment in the mirrors, so
that when the jar extinguishes direct light, the mirror
images are bright. This having been demonstrated, we
finally cover the top of the jar with a large quartz plate q.
The jar itself will now glow with all the rotational colours
as the polariser is rotated; differently-coloured images
being seen at the same time in the mirrors m, and the
whole forming a demonstration of indescribably delicate
beauty. It is convenient to have the mirrors hinged
together like a book, and to stand both jar and mirrors on
a circular piece of board, if no higher support be needed, in
order that the whole may be turned to face in succession all
sides of the room.
CHAPTER XXIII
HEAT
A NUMBEE of experiments showing the effects, nature, and
qualitative relations of heat are easily capable of projection.
Some specimen examples may suffice.
224. Expansion.—As regards the expansion of solids,
Gravesande’s ring and the various socket forms of apparatus
need no remark. Any ordinary form of pyrometer is readily
projected in action by the shadow method (§ 109) ; or a small