It is only
kept down by the pressure of the atmosphere above. We have a couple of
them; and if you take these two and press them together, you will see how
firmly they stick. And, indeed, we may use them as they are proposed to be
used, to stick against windows, or against walls, where they will adhere
for an evening, and serve to hang anything on that you want. I think,
however, that you boys ought to be shewn experiments that you can make at
home; and so here is a very pretty experiment in illustration of the
pressure of the atmosphere. Here is a tumbler of water. Suppose I were to
ask you to turn that tumbler upside-down, so that the water should not
fall out, and yet not be kept in by your hand, but merely by using the
pressure of the atmosphere. Could you do that? Take a wine-glass, either
quite full or half-full of water, and put a flat card on the top, turn it
upside-down, and then see what becomes of the card and of the water. The
air cannot get in because the water by its capillary attraction round the
edge keeps it out.
I think this will give you a correct notion of what you may call the
materiality of the air; and when I tell you that the box holds a pound of
it, and this room more than a ton, you will begin to think that air is
something very serious.
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