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.
Fig. 1. - EXPERIMENT UPON THE PERMEABILITY OF BODIES.
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.
Fig. 2. - PRINCIPLE OF THE HOT AIR BALLOON.