on a piece of paper. How remarkable it is that that thing which is light
enough to produce shadows of other objects, can be made to throw its own
shadow on a piece of white paper or card, so that you can actually see
streaming round the flame something which is not part of the flame, but is
ascending and drawing the flame upwards. Now, I am going to imitate the
sunlight, by applying the voltaic battery to the electric lamp. You now
see our sun, and its great luminosity; and by placing a candle between it
and the screen, we get the shadow of the flame.

[Illustration: Fig. 4.]

You observe the shadow of the candle and of the wick; then there is a
darkish part, as represented in the diagram, and then a part which is more
distinct. Curiously enough, however, what we see in the shadow as the
darkest part of the flame is, in reality, the brightest part; and here you
see streaming upwards the ascending current of hot air, as shewn by
Hooker, which draws out the flame, supplies it with air, and cools the
sides of the cup of melted fuel.

I can give you here a little further illustration, for the purpose of
shewing you how flame goes up or down; according to the current. I have
here a flame--it is not a candle flame--but you can, no doubt, by this
time, generalise enough to be able to compare one thing with another. What
I am about to do is to change the ascending current that takes the flame
upwards into a descending current. This I can easily do by the little
apparatus you see before me. The flame, as I have said, is not a candle
flame, but it is produced by alcohol, so that it shall not smoke too much.
I will also colour the flame with another substance[6], so that you may
trace its course; for with the spirit alone you could hardly see well
enough to have the opportunity of tracing its direction. By lighting this