Fig. 6.—The detached sphere floating under water.
Fig. 7.—The Centrifugoscope.
The Centrifugoscope.—I have here a toy, which we may suitably call the centrifugoscope, which shows in a simple way the formation of spheres of liquid in a medium of practically equal density. It consists of a large glass bulb attached to a stem, about three-quarters full of water, the remaining quarter being occupied by orthotoluidine. This liquid, being slightly denser than water at the temperature of the room, rests on the bottom of the bulb. When I hold the stem horizontally, and rotate it—suddenly at first, and steadily afterwards—a number of fragments are detached from the orthotoluidine, which immediately become spherical, and rotate near the outer side of the bulb. The main mass of the red liquid rises to the centre of the bulb, and rotates on its axis ([Fig. 7]), and we thus get an imitation of the solar system, with the planets of various sizes revolving round the central [pg 15] mass; and even the asteroids are represented by the numerous tiny spheres which are always torn off from the main body of liquid along with the larger ones. When the rotation ceases, the detached spheres sink, and after a short time join the parent mass of orthotoluidine. We can therefore take this simple apparatus at any time, and use it to show that a mass of liquid, possessing a free surface all round, and unaffected by gravity, automatically becomes a sphere. After all, this is only what we should expect of an elastic skin filled with a free-flowing medium.
Effect of Temperature on Sphere of Orthotoluidine.—I will now return to the large sphere formed under water in the flat-sided vessel, and direct your attention to an experiment which teaches an important lesson. By placing a little ice on the top of the water, we are enabled to cool the contents of the vessel, and we soon notice that the red-coloured sphere becomes flattened on the top and below, and sinks a short distance into the saline layer. Evidently the cooling action, which has affected both liquids, has caused the orthotoluidine to become denser than water. I now surround the vessel with warm water, and allow the contents gradually to attain a temperature higher than 75° F. You observe that the flattened drop changes in shape until it is again spherical; and as the heating is continued elongates in a vertical direction, and then rises to the surface, being now less dense than water. So sensitive are these temperature effects that a difference of 1 degree on either side of 75° F. causes a perceptible departure from the spherical shape in the case of a large drop. It therefore follows that [pg 16] orthotoluidine may be either heavier or lighter than water, according to temperature, and this fact admits of a simple explanation. Orthotoluidine expands more than water on heating, and contracts more on cooling. The effect of expansion is to decrease the density, and of contraction to increase it; hence the reason why warm air rises through cold air, and vice versa. Now if orthotoluidine and water, which are equal in density at 75° F., expanded or contracted equally on heating above or cooling below this temperature, their densities would always be identical. But inasmuch as orthotoluidine increases in volume to a greater extent than water on heating, and shrinks more on cooling, it becomes lighter than water when both are hotter than 75° F., and heavier when both are colder. We call the temperature when both are equal in density the equi-density temperature. Here are some figures which show how the densities of these two liquids diverge from a common value on heating or cooling, and which establish the conclusions we have drawn:—
Temperature. | Density. | ||
|---|---|---|---|
Deg. F. | Deg. C. | Water. | Orthotoluidine. |
50 | 10 | 0·9997 | 1·009 |
59 | 15 | 0·9991 | 1·005 |
68 | 20 | 0·9982 | 1·001 |
Equal: 75 | 24 | 0·9973 | 0·997 |
86 | 30 | 0·9957 | 0·992 |
95 | 35 | 0·9940 | 0·988 |
104 | 40 | 0·9923 | 0·983 |
[pg 17]
Fig. 8.—Aniline drops falling through cold water and ascending through hot water.
Other Examples of Equi-Density.—There are many other liquids which, like orthotoluidine, may be heavier or lighter than water, according to temperature, and I now wish to bring to your notice the remarkable liquid aniline, which falls under this head. Aniline is an oily liquid, which, unless specially purified, has a deep red colour. It forms the basis of the beautiful and varied colouring materials known as the aniline dyes, which we owe to the skill of the chemist. The equi-density temperature of water and aniline is 147° F. or 64° C.; that is, aniline will sink in water if both be colder than 147° F., and rise to the surface if this temperature be exceeded. We may illustrate this fact by a simple but striking experiment. Here [pg 18] are two tall beakers side by side, and above them a cistern containing aniline ([Fig. 8]). The stem of the cistern communicates with the two branches of a horizontal tube, the termination of one branch being near the top of one of the beakers, whilst the other branch is prolonged to the bottom of the second beaker, and is curved upwards at the end. Both branches are provided with taps to regulate the flow of liquid, and to commence with are full of aniline. Cold water is poured into the beaker containing the shorter branch until the end is submerged; and water nearly boiling is placed in the second beaker to an equal height. I now open the taps, so that the aniline may flow gradually into each beaker; and you notice that the drops of aniline sink through the cold water and rise through the hot. We have thus the same liquid descending and ascending simultaneously in water, the only difference being that the water is cold on the one side and hot on the other. Prolonging the delivery-tube to the bottom of the beaker containing the hot water enables the rising drops to be observed throughout the length of the column of water; and in addition enables the cold aniline from the cistern to be warmed up on its way to the outlet, so that by the time it escapes its temperature is practically the same as that of the water. If this temperature exceed 147° F., the drops will rise. We might, in this experiment, have used orthotoluidine instead of aniline; or, indeed, any other liquid equal in density to water at some temperature intermediate between those of the hot and cold water—always provided that the liquid chosen did not mix with water. Amongst such other liquids may be [pg 19] mentioned anisol; butyl benzoate; and aceto-acctic ether; but none of these possess the fine colour of aniline or its chemical relative orthotoluidine, and in addition are more costly liquids. Besides these are a number of other liquids rarer still, practically only known to the chemist, which behave in the same way. These liquids are all carbon compounds, and more or less oily in character. There is a simple rule which may be used to predict whether any organic liquid will be both lighter and heavier than water, according to temperature. 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.