Here it is: If the density of the liquid at 32° F. or 0° C. be not greater than 1·12, the liquid will become less dense than water below 212° F. or 100° C., at which temperature water boils. This rule is derived from a knowledge of the extent to which the expansion of organic liquids in general exceeds that of water. I have considered it necessary to enter at some length into this subject of equi-density, as much that will follow involves a knowledge of this physical relation between liquids. *Aniline Films or Skins.*—We have previously concluded, largely from circumstantial evidence, that a liquid drop is encased in a skin or what is equivalent to a skin, and I propose now to show by experiments with aniline how we can construct a drop, commencing with a skin of liquid. Here is some aniline in a vessel, covered by water. I lower into the aniline a circular frame of wire, which I then raise slowly into the overlying water; and you observe that a film of aniline remains stretched across the frame. By lifting the frame up and down in the water the skin is stretched, forming a drop which is constricted near the frame (Fig. 9). On lifting the wire more suddenly, the skin of aniline closes in completely at the narrow part, and a sphere of water, encased in an aniline skin, then falls through the water in the beaker, and comes to rest on the aniline below—into which, however, it soon merges. You were previously asked to regard a drop of liquid as being similar to a filled soap-bubble; and this experiment realizes the terms of the definition. And it requires only a little imagination to picture a drop surrounded by its own skin instead of that of another liquid. It is easy to make one of these enclosed water-drops by imitating the blowing of a soap-bubble—using, however, water instead of air. In order to do this I take a piece of glass tubing, open at both ends, and pass it down the vessel, until it reaches the aniline. Water, in the meantime, has entered the tube, to the same height as that at which it stands in the vessel. On raising the tube gently, a skin of aniline adheres to the end; and as we raise it still further, the water in the tube, sinking so as to remain at the level in the vessel, expands the skin into a sphere (Fig. 9)—the equivalent of a filled soap-bubble. On withdrawing the tube gradually, the composite sphere is left hanging from the surface of the water. [Illustration: __Fig._ 9.—Aniline skins enveloping water._]