pull, whereas B and C can only exert their strength at an angle, and the larger the angle the more they are handicapped. If A be more powerful than B and C, the globule will spread; but the result of the spreading is to diminish the angles at which the pulls of B and C are inclined to the surface, and hence their effective opposition to A will be increased. Moreover, the spreading of the liquid diminishes the surface tension of the water—that is, weakens A—and hence it becomes possible for B and C to prevail and draw back the surface of the globule which A had previously stretched. If, in spite of these disabilities, A should still be the stronger, the globule will be stretched until it covers the whole surface; whereas if B and C overcome A, the globule will shrink, increasing the angles at which B and C operate, and therefore reducing their effective pulls, until their combined strength is equal to that of A, when the globule will remain at rest. Bearing these facts in mind, we can understand why a small drop of oil placed on a clean water surface spreads across; for in this case A is stronger than B and C combined. But when the surface of the water is covered with a layer of oil, A is weakened, and can no longer overcome the opposing pulls of B and C. Hence a further drop of oil poured on to the surface remains in the form of a globule. *Movements due to Solubility.*—When small fragments of camphor are placed on the surface of water some remarkable movements are seen.³ The bits of camphor move about with great rapidity over the surface, and generally, in addition, show a rapid rotary motion. The explanation usually given is that the camphor dissolves in the water at the points of contact forming a solution which possesses a less surface tension than pure water. This solution is in consequence stretched by the tension of the rest of the surface, and the camphor floating on its solution is therefore made to move in the direction of the line along which the stretching force happens to be the greatest. But the camphor continues to dissolve wherever it goes, and is therefore continuously pulled about as a result of this interplay of tensions. Touching the surface with a wire which has been dipped in oil immediately arrests the movements, owing to the tension of the water being diminished to such an extent by the skin of oil that it is no longer competent to stretch the part on which the camphor floats. No doubt this explanation is correct so far as it goes, but it is highly probable that when the floating substance dissolves, other forces are called into action in addition to the tensions. ³ These movements were first recorded by Romieu in 1748 and were ascribed by him to electricity. [Illustration: __Fig._ 36.—Aniline globules on a water surface._]