as the
sparks would yield. This instrument has been very kindly lent to me by
Professor Dewar. I will project a picture of the apparatus on the
screen, so that those at a distance may be better able to see the
construction of the instrument.

[Illustration: Fig. 22.]

And now follow me carefully. I take the steel and the flint, and
striking them together I get sparks. I want you to ask yourselves, Where
do the sparks come from? Each spark is due to a minute piece of _iron_
being knocked off the steel by the blow of flint with steel. Note the
precise character of the spark. Let me sprinkle some iron filings into
this large gas flame. You will notice that the sparks of burning iron
filings are very similar in appearance to the spark I produce by the
collision of my flint and steel.

[Illustration: Fig. 23.]

But now I want to carry you somewhat further in our story. It would not
do for me simply to knock off a small piece of iron; I want when I knock
it off that it should be red-hot. Stay for a moment and think of
this--iron particles knocked off--iron particles made red-hot. All
mechanical force generates heat.[A] You remember, in my last lecture, I
rubbed together some pieces of wood, and they became sufficiently hot to
fire phosphorus. On a cold day you rub your hands together to warm them,
and the cabmen buffet themselves. It is the same story--mechanical force
generating heat! The bather knows perfectly well that a rough sea is
warmer than a smooth sea. Why?--because the mechanical dash of the waves
has been converted into heat. Let me remind you of the familiar phrase,
"striking a light," when I rub the match on the match-box. "Forgive me
urging such simple facts by such simple illustrations and such simple
experiments. The facts I am endeavouring to bring before you are
illustrations of principles that determine the polity of the whole
material universe." Friction produces heat. Here is a little toy
(cracker) that you may have seen before (Fig. 23). It is scientific in
its way. A small quantity of fulminating material is placed between two
pieces of card on which a few fragments of sand have been sprinkled
(Fig. 23 _a_). The two ends of the paper (_b b_) are pulled asunder. The
friction produces heat, the heat fires the fulminate, and off it goes
with a crack. And now put this question to yourselves, What produced the
friction? Force. What is more, the amount of heat produced is the exact
measure of the a