_The Porosity and Permeability of Bodies._--Take two tumblers of the same
size, place one of them upon a table, and pour into it a small quantity
of nearly boiling water. Cover this glass with a sheet of cardboard, and
invert the other one upon it. This second tumbler must be previously
wiped so as to have it perfectly dry and transparent. In a few seconds
the steam from the lower tumbler will traverse the cardboard (which will
thus exhibit its permeability), and will gradually fill the upper
tumbler, and condense and run down its sides. Wood and cloth may be
experimented with in succession, and will give the same results; but
there are other substances that are _impermeable_, and will not allow
themselves to be traversed. Such, for example, is the vulcanized rubber
of which waterproofs are made. This experiment explains to us why fog
is, as has been well said, so _penetrating_. It traverses the tissue of
our overcoat and of our flannel, and comes into contact with our body. On
the contrary, a rubber coat preserves us against its action.
[Illustration: Fig. 1.--EXPERIMENT UPON THE PERMEABILITY OF BODIES.]
_A Hot Air Balloon_.--Make a hollow cylinder of small diameter out of a
sheet of paper such as is used for cigarette packages, and turn in the
ends slightly so that it shall preserve its form. If the cylinder seems
too difficult to make, a cone may be substituted. Now set fire to the
cylinder or cone at its upper part. The paper will burn and become
converted into a thin sheet of ashes, which will contract and curl
inward. This light residuum of ashes, being filled with air rarefied by
combustion, will suddenly rise to a distance of two or three yards. Here
we have a Montgolfier balloon.--_La Nature_.
[Illustration: Fig. 2.--PRINCIPLE OF THE HOT AIR BALLOON.]
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