Page:Popular Science Monthly Volume 9.djvu/607

Rh If we could obtain a body the weight of which was precisely the same as the weight of the water it displaced, it would have no tendency to sink or swim, but would remain at rest in any part of the water into which we might choose to place it. Hence this body would be practically free from the attractive influence of the earth, and we should have succeeded in neutralizing the force of gravity, since a body having no tendency to rise or fall might be considered as removed to such a distance from the earth as to have no weight. Of course, this conclusion is independent of the fact whether the body introduced into the water is a liquid or a solid, and we may substitute for the water any other liquid; but, if we employ two liquids, they must satisfy the following conditions: In the first place, they must not mix together, as wine and water do, but must remain separate, like water and oil. In the second place, the weight of any volume of one must be exactly equal to that of the same volume of the other; and, in the third place, the two liquids must have, when in contact. no chemical effect upon each other. Could two such liquids be found, a small quantity of the one introduced into a mass of the second would be a state eminently favorable for determining the shape which it would assume under the influence of its surface-tension alone. It would, as I have pointed out, be free from the attraction of the earth, and it would also be free from the force of adhesion to the sides of a solid vessel. It would, however, be extremely difficult to find two liquids which would satisfy these conditions; but, although we cannot find them to our hand, we are able to manufacture them. Water is a liquid which is heavier than oil, and alcohol is on the other hand lighter than oil; and, if we mingle water and alcohol, we may make a mixture, the weight of any given volume of which is precisely equal to that of the same volume of oil, and by introducing a few drops of oil into the mass of alcohol and water of the right density we ought to succeed in observing the form which a liquid assumes under the influence of its surface-tension alone. You now see upon the screen the image of a mass of oil in a mixture of alcohol and water of the kind I have just described; and you see that our question is at once answered—the oil assumes a spherical form. From this we learn that a liquid, if left to the action of its surface-forces alone, will become a sphere. But inasmuch as the effect of the attractive influence of the earth, or the weight of the liquid, increases with the quantity we use, while on the other hand the surface-tension, or its own moulding molecular force, remains precisely the same, we should, if we use a large quantity of liquid, expect the weight to be the particular force which determined its shape; and if we employ a small mass of liquid, then the surface-tension, growing proportionately greater, would become the more important. Thus it follows that, although we have to use the most accurate adjustment in order to obtain a sphere of oil an inch in diameter, every rain-drop, every