homogeneous medium, he takes the envelope itself—a drop void of water, or rather full of air—represented for convenience of manipulation by a soap-bubble, and consisting of two films separated by an extremely thin mass of water. The pressure is the same in every part, and the curvature uniform, and that which gives the least possible surface—a sphere. The pressure is strong enough to drive tobacco-smoke back through a pipe-stem or to blow out a candle. The curved film may be deformed by passing it through rigid frames, but it will always preserve a geometrical shape, for it can not continue to exist except upon the condition of exercising an equal pressure throughout upon the air imprisoned within it; but some of the shapes it will assume within this rule are very curious.
If a drop of water is poured upon another liquid, it is still imprisoned in its contractile sac, but in one having two walls of unequal elasticity; the upper wall resting against the air, and the lower one against the liquid. The line of suture of these two
Fig. 3.—A Paper Box closing upon itself when Water is poured into it.
walls floats in three different media—air, water, and the subjacent liquid; or, to use M. Gossart's figure, it is like a cord drawn by three different forces, which are represented in this case by the upper and lower walls of the sac and the uncovered membrane of the inferior liquid, pulling against one another, as when three ropes are pulled by three men of unequal strength. Suppose, as the extreme case, that the attraction of the membrane exterior to the drop so prevails over the tension of the two walls of the sac that they can not rest in equilibrium. Then the sac will be drawn
