*The “Diving” Drop.*—In order to illustrate the tension at the boundary surface of two liquids, I now show an experiment in which a drop is forcibly projected downwards by the operation of this tension. I pour some water into a narrow glass vessel, and float upon it a liquid called _dimethyl-aniline_, so as to form a layer about 1 inch in depth. A glass tube, open at both ends, is now passed down the floating liquid into the water, and then raised gradually, with the result that a skin of water adheres to the end, and is inflated by the upper liquid, forming a sphere on the end of the tube (Fig. 10). On withdrawing the tube from the upper surface, the sphere is detached and falls to the boundary surface, where it rests for a few seconds, and is then suddenly shot downwards into the water (Figs. 11 and 12). It then rises to the interface; breaks through, and mingles with the floating liquid, thereby losing its identity. Why should the drop, which is less dense than water, dive below in this manner? The explanation is that the drop (which consists of a skin of water filled with dimethyl-aniline), after resting for a time on the joining surface, loses the under part of its skin, which merges into the water below. The shape of the boundary of the two liquids is thereby altered, the sides now being continuous with the skin forming the upper part of the drop. This is an unstable shape; and accordingly the boundary surface flattens to its normal condition, and with such force as to cause the drop beneath it to dive into the water, although the liquid is lighter than water and tends to float. The result is the same as that which would occur if a marble were pressed on to a stretched disc of rubber, and then released, when it would be projected upwards owing to the straightening of the disc. I now repeat the experiment, using paraffin oil instead of dimethyl-aniline; but in this case the drop is only projected to a small depth, and the effect is not so marked. The experiment furnishes conclusive evidence of the existence of the interfacial tension.