There is one feature common to the spheres which compose a mist or fog, or indeed to any kind of drop resulting from the condensation of moisture in the atmosphere. As shown by the deeply interesting researches of Aitken and others, each separate sphere forms round a core or nucleus, which is usually a small speck of dust, and hence an atmosphere charged with solid particles lends itself to the formation of dense fogs immediately the temperature falls below the dew-point. But dust particles are not indispensable to the production of condensed spheres, for it has been shown that the extremely small bodies we call “ions,” which are electrically charged atoms, can act as centres round which the water will collect; and much atmospheric condensation at high elevations is probably due to the aid of ions.[2] Near the surface, however, where dust is ever present, condensation round the innumerable specks or motes is the rule. [pg 54] Here, for example, is a jet of steam escaping into air, forming a white cloud composed of a multitude of small spheres of condensed water. If now I allow the steam to enter a large flask containing air from which the dust has been removed by filtration through cotton wool, no cloud is formed in the interior, but instead condensation takes place at the end of the jet, from which large drops fall, and on the cold sides of the flask. The cloud we see in dusty air is entirely absent, and the effect of solid particles in the process of condensation is thus shown in a striking manner. Clouds are masses of thick mist floating at varying heights in the atmosphere. On sinking into a warmer layer of dry air the particles of which clouds are composed will evaporate and vanish from sight. If the condensation continue, however, the spheres will grow in size until the friction of the atmosphere is unable to arrest their fall; and then we have rain. And whether the precipitation be very gentle, and composed of small drops falling slowly, as in a “Scotch mist,” or in the form of rapid-falling large drops such as accompany a thunderstorm, the processes at work are identical. Every particle of a mist or cloud, and every raindrop, is formed round a nucleus, and owes its spherical shape to the tension at the surface.
Liquid Clouds in Liquid Media.—Just as the excess of moisture is precipitated from saturated air when the temperature falls, so is the excess of one liquid dissolved in another thrown down by cooling below the saturation temperature. Moreover, a liquid when precipitated in a second liquid appears in the form of myriads of small spheres, which have the [pg 55] appearance of a dense cloud. Here is some boiling water to which an excess of aniline has been added, so that the water has dissolved as much aniline as it can hold. Aniline dissolves more freely in hot water than in cold, so that if I remove the flame, and allow the beaker to cool, the surplus of dissolved aniline will settle out. Cooling takes place most rapidly at the surface, and you observe white streaks falling from the top into the interior, where they are warmed up and disappear. Soon, however, the cooling spreads throughout; and now the streaks become permanent, and the water becomes opaque, owing to the thick white cloud of precipitated aniline. The absence of the red colour characteristic of aniline is due to the extremely fine state of division assumed in the process. If left for some hours, the white cloud sinks through the water to the bottom of the beaker, where the small particles coalesce and form large drops, leaving the overlying water quite transparent. The process is quite analogous to the precipitation of moisture from the atmosphere in the form of small spheres, which, if undisturbed, would gradually sink to the ground and leave the air clear.
Overheated Drops.—The temperature at which a liquid boils, under normal conditions, depends only upon the pressure on its surface. Thus water boiling in air, when the pressure is 76 centimetres or 29·92 inches of mercury, corresponding to 14·7 pounds per square inch, possesses a temperature of 100° C. or 212° F. At higher elevations, where the pressure is less, the boiling point is lower; thus Tyndall observed that on the summit of the Finsteraarhorn (14,000 feet) water [pg 56] boiled at 86° C. or 187° F. Conversely, under increased pressure, the boiling point rises; so that at a pressure of 35 pounds per square inch water does not boil until the temperature reaches 125° C. or 257° F. There are certain abnormal conditions, however, under which the boiling point of a liquid may be raised considerably without any increase in the pressure at the surface; and it is then said to be “over-heated.” Dufour showed that when drops of water are floating in another liquid of the same density, they may become greatly overheated, and if very small in size may attain a temperature of 150° C. or 302° F., or even higher, before bursting into steam. In order to provide a medium in which water drops would float at these temperatures, Dufour made a mixture of linseed oil and oil of cloves, which possessed the necessary equi-density temperature with water. To demonstrate this curious phenomenon, I take a mixture of 4 volumes of ethyl benzoate and 1 volume of aniline, which at 125° C. or 257° F. is exactly equal in density to water at the same temperature. I add to the mixture two or three cubic centimetres of freshly-boiled water, the temperature being maintained at 125° C. by surrounding the vessel with glycerine heated by a flame. At first the water sinks, but on attaining the temperature of the mixture it breaks up with some violence, forming spheres of various sizes which remain suspended in the mixture. Any portion of the water which has reached the surface boils vigorously, and escapes in the form of steam; and some of the larger spheres may be observed to be giving off steam, which rises to the surface. Most of the spheres, however, remain [pg 57] perfectly tranquil, in spite of the fact that the water of which they are composed is many degrees above its normal boiling point. If I penetrate one of these spheres with a wire, you notice that it breaks up immediately, with a rapid generation of steam. A complete explanation of this abnormal condition of water is difficult to follow, as a number of factors are involved. One of the contributory causes—though possibly a minor one—is the opposition offered to the liberation of steam by the tension at the surface of the spheres.
Fig. 34.—Spheroid of water on a hot plate.
Floating Drops on Hot Surfaces.—If a liquid be allowed to fall in small quantity on to a very hot solid, it does not spread out over the surface, but forms into drops which run about and gradually evaporate. By careful procedure, we may form a very large, flattened drop on a hot surface, and on investigation we shall notice some remarkable facts. I take a plate of aluminium, with a dimple in the centre, and make it very hot by means of a burner. You see the upper surface of this plate projected on the screen. I now allow water to fall on the plate drop by drop, and you hear a hissing noise produced when each drop strikes the plate. The separate drops gather together in the depression at the centre of the plate, forming a very large flattened globule. You might have expected the water to boil vigorously, but no signs of ebullition are visible; and what is more remarkable, the temperature of the drop, in spite of its surroundings, is actually less than the ordinary boiling point. Notice now how the drop has commenced to rotate, and has been set into vibration, causing the edges to become scalloped ([Fig. 34]). The drop, although not actually [pg 58] boiling, is giving off vapour rapidly, and therefore gradually diminishes in size. And now I want to prove that the drop is not really touching the plate, but floating above it. To do this I make an electric circuit containing a cell and galvanometer, and connect one terminal to the plate and place the other in the drop. No movement is shown on the galvanometer, as would be the case if the drop touched the plate and thus completed the electric circuit. And at close range we can actually see a gap between the drop and the plate, so that the evidence is conclusive. If now I remove the flame—leaving the electric circuit intact—and allow the plate to cool, we notice after a time that the globule flattens out suddenly and touches the plate, as shown by the deflection of the galvanometer; [pg 59] and simultaneously a large cloud of steam arises, due to the rapid boiling which occurs immediately contact is made.
What we have seen in the case of water is shown by most liquids when presented to a surface possessing a temperature much higher than the boiling point of the liquid. A liquid held up in this manner above a hot surface is said to be in the spheroidal state, to distinguish it from the flat state usually assumed by spreading when contact occurs between the liquid and the surface. It is doubtful whether any satisfactory explanation of the spheroidal state has ever been given. Evidently, the layer of vapour between the plate and the drop must exert a considerable upward pressure in order to sustain the drop, but the exact origin of this pressure is difficult to trace.
[pg 60]
LECTURE III
Spreading of Oil on the Surface of Water.—If a small drop of oil be placed on the surface of water it will be observed to spread immediately until it forms a thin layer covering the surface. If a further addition of the oil be made, globules will be formed, which, as you now see upon the screen, remain floating on the surface. The spreading of the oil in all directions from the place on which the small quantity of oil was dropped is due to the superior surface tension of water, which pulls the oil outwards. The surface tension of the oil opposes that of the water, and would prevent the drop from spreading were it not overcome by a greater force. The result is the same as would be observed if the centre or any other part of a stretched rubber disc were weakened; the weak part would be stretched in all directions, and the rest of the disc would shrink towards the sides. When the oil has spread out, however, and contaminated, as it were, the surface of the water, the surface tension is reduced, and is not sufficiently strong to stretch out a further quantity of oil, which, if added, remains in the form of a floating globule.