Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/764

Rh 728 EVAPORATION pressure of its vapour is less than that to which the liquid is exposed, evaporation will go on at its free surface only ; but if the temperature is raised so that the pressure of the vapour is greater than that exerted upon the liquid, bubbles of vapour can exist within the liquid itselfc, and if once formed will rise through the liquid and escape at the surface. This phenomenon is called ebullition or boiliny ; and the temperature at which the pressure of the vapour of a substance is equal to the standard atmospheric pressure is called its boiling-point. The standard atmospheric pressure generally adopted is that exerted by a column of mercury 760 millimetres in height at C. at the sea level in latitude 45. This is equivalent to about 29 905 inches of mercury at C. at the sea-level in the latitude of London. The pressure of a megadyne per square centi metre has been proposed as the standard atmosphere, but this has not yet been generally adopted. When a quantity of water is heated from the lower surface, the water near the bottom is at a higher tempera ture than the superincumbent layers, and the bubbles of steam formed there on rising are surrounded by water at a temperature below the boiling-point, and, being conse quently unable to sustain the pressure to which they are exposed, they collapse with a slight sound. These sounds repeated in rapid succession constitute the &quot; singing &quot; of the kettle, and are exchanged for a very much softer sound when the whole of the water reaches the boiling- point, and steam bubbles escape from the surface. Though bubbles of pure steam once produced can exist under atmospheric pressure if the temperature be above the boiling- point, yet such bubbles will not necessarily be produced in pure water as soon as it reaches that temperature. If water which has been carefully freed from air by long boiling be heated in a clean glass vessel, its temperature may be raised considerably above the boiling-point; but as soon as the continuity of the water is broken by the formation of a bubble of steam, ebullition ensues with explosive violence, and the temperature falls nearly to the boiling-point. Drops of water suspended in a mixture of linseed oil and oil of cloves of the same specific gravity have been heated by Dufour to 180 C., and generally fatty oils poured on the surface of water tend to prevent ebullition. It has been stated that the boiling of pure water has not yet been observed. Certain solutions, especially strong solutions of caustic alkalies, are very liable to an explosive evolution of steam at intervals, and the best way of preventing it is the introduction, when possible, of a small piece of a metal which can decompose water. Though the temperature at which water boils depends on the impurities which it contains, and the nature of the vessel in which it is placed, yet the temperature of the steam above the water depends only on the pressure. This has been long acknowledged when the quantity of impurity dissolved in the water is small, and in order to determine the boiling-points upon thermometers they are immersed in the steam above boiling water without allowing their bulbs to touch the water. When the quantity of salt dissolved is very great, the temperature of the boiling solution is generally very much above the boiling-point of water. Thus, according to Faraday, saturated solutions of common salt, nitre, and potassic carbonate boil at 109, 115-6, and 140 3 C. respectively. The temperature of ebullition of a saline solution is sometimes employed to determine the percentage of salt present. Notwithstanding the high temperature of the solution, it seems that the temperature of the steam when first liberated from the solution is the same as that produced by water boiling at the same pressure. This conclusion is supported by Dufour, though Magnus and some others were of a different opinion. If a thermometer with a clean unpro tected bulb be immersed in the steam above a concentrated saline solution boiling at ordinary pressure, its temperature will quickly rise to 100 C., then become almost stationary, and afterwards slowly rise to a temperature somewhat below that of the liquid, and depending on its nearness to the solution and the facilities which are offered for the escape of heat from the bulb. On removing the thermo meter and allowing it to cool, there will generally be found a quantity of salt sticking to the bulb which has been splashed upon it from the solution. If the bulb of a ther mometer be covered with cotton which has been sprinkled with some salt, and be then immersed in steam, whether above a saline solution or above boiling water, its tem perature will quickly rise considerably above the boiling- point, and several thermometers whose bulbs have been covered with different salts will indicate different tempera tures if suspended side by side in the same vessel of steam, leading us to suspect that the high temperature recorded by the thermometer above the saline solution may be due in part at least to salt which has been splashed upon the bulb. If the bulb be protected from splashes by a metal screen placed below it, and from condensed water trickling down the stem by a guard placed above it, the temperature will at once rise to 100 C.; but the further rise of temperature will be so slow that it may be accounted for by the radiation from the liquid and from the metal screen, which of course becomes heated in the same way as a naked thermometer placed in its position. If a test tube con taining mercury be immersed in the solution, and the thermometer bulb placed in it till it reaches the same temperature, on raising it into the steam the temperature will be seen to fall considerably. If a small quantity of a liquid be placed in a metal vessel whose temperature has been raised very much above the boiling-point of the liquid, vapour will be produced so rapidly from the under surface of the liquid that it will be supported on a cushion of its own vapour, and thus prevented from coming into contact with the metal, the separation being so complete that if the liquid be an electrolyte a current from an ordinary battery cannot be made to pass from the liquid to. the metal. This condition of the liquid is called the spheroidal state, and is often referred to as Leidenfrost s phenomenon. It may fre quently be noticed that the drop is in a state of rapid rota tion. If by any means an indentation is made in the surface of the drop, vibrations will be set up in it, causing the horizontal section to pass into the form of a curvilinear polygon, in the same manner as the edge of a bell changes its form when struck. The sarface of the drop then presents a &quot; beaded &quot; or corrugated appearance, formed by the superposition of the retinal images of the drop in the two extreme conditions which it assumes, and there fore always presenting an even number of corrugations corresponding to the vibrating segments. Surface tension of course supplies the forces necessary to produce the vibrations. When a ventral segment projects beyond the mean surface of the drop so as to form a &quot; bead,&quot; more surface is exposed by it to the heating actien of the metal than when it is in its mean position, and when it lies within the mean, or spheroidal, surface so as to form a &quot;flute,&quot; less surface is exposed by it ; but as the generation of steam cannot be instantaneous, more steam will escape from the segment while it is receding towards the centre than while it is advancing, and thus the pressure of the escaping steam upon each ventral segment will vary with the phase of vibration in such a manner as to supply the energy necessary to the continuance of the motion. If the drop be examined by ordinary daylight a fluted ontlim can be distinctly seen within the beaded outline, but if i