Condensation

“ Condensation, as we have seen, results from a cooling of the air. When vapor is condensed, it does not at once take the form of running water. The cold glass brought into the warm room has first a fine film of mist formed upon it, and then by degrees the clear drops of water come. In reality mist is made up of exceedingly minute particles of water, and it is the running together of these which makes the larger drops. So in nature on the great scale, when condensation occurs the vapor first appears as a fine mist. ”

“ It is undoubtedly the case that mingled with these little bubbles of moisture there are fine particles of solid water hanging on and carried along with them. Undoubtedly this is true; at least just before the final act of condensation takes place; and when the little hollow spherules collapse they are gathered together in drops of water larger or smaller according to the rapidity of condensation. [Pg 70] There is probably another power at work to prevent the too ready precipitation of moisture when condensed, and that is the wind. ”

“ It is mainly a summer condensation, especially high fog. It has been attributed to the cooling effect of an atmosphere colder than the earth, but it often occurs when the earth is the coldest, and when the vapor, as it rises, is colder than the air, and could not give out heat to a warmer medium. (See American Journal of Science, vol. xliv. p. 40.) Again, it is not mere condensation, but a formation of globules or vesicles, hollow, and the air expanded in them, by means of which they float like a soap bubble which contains the warm air of the breath. ”

“ There always has to be a beginning, and that occurs wherever the conditions are most favorable for condensation of vapor. If the earth is wet and the sun is hot the evaporation may be very rapid as well as the ascent of the invisible moisture, which carries with it the air, which in turn expands the higher it rises, thus producing cold. This, taken with the normal cold that exists in the higher regions, may be sufficient to produce a sudden condensation of this ascending vapor, which is all that is necessary to form a cloud. ”

“ Now, in a climate such as our own, where the lower regions of the air contain large quantities of water vapour, any considerable rise brings about more or less condensation, and that condensation is attended by a liberation of very large quantities of heat, which retard cooling in the ascending current, and so facilitate the production of instability. ”

“ Possibly the difficulty may be met by taking account of the enormous amount of condensation which must be going on within the photosphere. To supply the heat which the sun throws off (enough to melt each minute a shell of ice nearly forty feet thick over his entire surface) would require the condensation of enough vapor to make a sheet of liquid five feet thick in the same time—supposing, that is, the latent heat of the solar vapors not greater than that of water vapors. ”

“ As water in its condensed state is 815 times heavier than air, the question naturally comes to one why it does not immediately fall to the earth when it condenses. There are at least two and probably more stages of condensation. Investigators into the phenomenon of cloud formation claim to have ascertained that the first effect of condensation is to form little globes of moisture that are hollow, like a bubble, with very thin walls. Everyone has recognized the ease with which a soap bubble will float in the air, and yet it is simply a film of moisture. ”

“ Further, when condensation occurs, an amount of heat is evolved, as temperature, exactly equal to that which was rendered latent in the evaporation of the same substance under the same pressure; and, in like manner, when chemical re-combination of dissociated elements occurs, an amount of heat is evolved, as temperature, exactly equal to that which disappeared when the13 compound was dissociated by heat alone under the same pressure. ”

“ Condensation, whether of air by pressure, or of metals by percussion, is another powerful source of heat. Thus, iron may be so dexterously hammered as to become red-hot, and the rapid condensation of a confined portion of air will set tinder on fire.
(348.) The most violent heats known are produced314 by the concentration of the solar rays by burning glasses,—by the combustion of oxygen and hydrogen gases mixed in the exact proportion in which they combine to produce water,—and by the discharge of a continued and copious current of electricity through a small conductor. ”

“ When a cooler stratum of the upper air sweeps down nearer the earth than usual, bringing with it the dew-point, condensation takes place so near the surface, that the rain-drops have not time to cool, at the same time that an immense quantity of latent heat has been liberated in the act of condensation. ”

“ When we consider that in converting a given volume—say, one cubic metre—of water into vapour, enough heat is consumed to lower about 1,650,000 cubic metres of air by 1° C. in temperature, and that the same amount of heat is liberated when the vapour so produced returns to the liquid state, we perceive how powerfully the ordinary processes of evaporation and condensation must affect the temperature of the air. ”

“ As it ascends the air becomes cooler, and yet retains its water vapour. When the rising currents are large they mix little with the descending dry air, and on reaching a certain level condensation [127] takes place, and we have the beginning of a cumulus. ”

“ We thus see that whirlwinds of great violence occur when the air is dry, and there can be no condensation. When, however, they are formed at sea, and occasionally on land, the air next the surface is saturated with moisture; and this moisture is condensed when it is carried to a great height, forming clouds, or falling in showers of rain and hail. This condensation of vapour is an effect, and not a cause, and takes place, not in the centre, but at the top or at the sides of the ascending column. ”

“ Still, however, the entire quantity of heat furnished by the body to the gas during its dilatation by contact with it, passes into the body during the condensation of the gas, which takes place by contact with it. ”

“ How unphilosophical, too, to talk of latent heat in connection with evaporation, at the lowest temperature known. Meteorologists must revise their opinions on the subject of condensation. This latent heat has never been actually met with; on the contrary, the most sudden and complete condensations of the vapor of the atmosphere are attended by as sudden and extraordinary productions [Pg 318] of cold, and consequent hail, and the connection between condensation and electricity is shown by too many facts to permit the old theory to stand.
Fog never forms with the thermometer below 32°. ”

“ The reciprocal transmission of vapour to the liquid state is called condensation, and the vapour is said to be condensed.
(p. 28) We shall now examine more minutely the circumstances which attend these remarkable and important changes in the state of body.
(11.) Let us suppose that a thermometer is imbedded in any solid body; for example, in a mass of sulphur; and that it stands at the ordinary temperature of 60 degrees: let the sulphur be placed in a vessel, and exposed to the action of fire. ”

“ Heat.—The most familiar of the active agencies that resist condensation is heat. Upon this the existing volume of the earth is immediately dependent, in some large part at least. As this heat is dissipated, the earth shrinks. This shrinkage increases the force of gravity, and hence the internal pressure increases, and, if further compression takes place as the result of this increased pressure, additional heat is developed, which checks further condensation until it is dissipated. ”

“ A greater proportion of emanation condenses the longer the time allowed for condensation under the same conditions. The condensation occurs more rapidly in hydrogen than in oxygen, as the diffusion is greater in the former gas. For the same reason the condensation occurs faster the lower the pressure of the gas present. Finally, when the emanation is carried by a steady stream of gas, a smaller proportion condenses than in the other cases, because the concentration of emanation particles per unit volume of gas is less under these conditions. ”

“ This condensation on every side of the mountains would leave a partial vacuum which would set up currents from every direction to restore the equilibrium, thus bringing in more super-saturated air to suffer condensation and add its supply of water, again increasing the in-draught of more air. ”

“ By losing heat it contracts, but the heat generated by the contraction is more than sufficient to keep the temperature from falling. A gaseous mass, losing heat by radiation, must, therefore, at the same time, grow both smaller and hotter, until the density becomes so great that the ordinary laws of gaseous expansion reach their limit, and condensation into the liquid form begins. ”

“ This condensation is dew. It always forms when the conditions are right, summer and winter. In cold weather we call it frost. It has been stated in a former chapter on evaporation that the capacity of the air for holding moisture in a transparent form depends upon its temperature. If the temperature is at the freezing point it will contain the 160th part of the atmosphere's own weight as aqueous vapor. If it is 60 degrees Fahrenheit the air will retain six grains of transparent moisture to the square foot of air, while at 80 degrees it will contain nearly eleven grains. ”

“ The condensation of vapour is for one what the consumption of fuel is for the other; only with the winds the same heat may be used over and over again, and for many purposes. By simply sending moist air to the top of snow-capped mountains, condensing its moisture, and bringing it down to the surface again, it is made hot. Though by going up the air be cooled, it is expanded, and receives as sensible heat the latent heat of its vapour ”

“ The result is that from the time the gaseous mixture leaves the retort it begins to deposit condensation products owing to the decrease in temperature. Condensation takes place in the ascension pipe, in the arch piece leading to the hydraulic main, and to a still greater extent in the hydraulic main itself where the gas has to pass through water. ”

“ The formation of cumulo-nimbus and cumulus is dependent upon the presence of a large amount of water vapour. It is worth while to consider whether the atmospheric movements which bring about the condensation could exist without moisture. Wherever we find differences of temperature between neighbouring places we must get [116] currents of hot air rising from the warmer spots, and compensating descending currents around them. ”

“ Gases in many cases undergo a remarkable condensation when chemically combined. In slaking lime, the water becomes solid in the molecules of the hydrate of lime formed, and the intense heat produced arises from the liberation of that caloric which had been employed to keep the water liquid. When a solid passes into the liquid state, cold is produced by the abstraction from surrounding objects of the heat required to effect fluidity. An alteration of temperature occurs whenever chemical change takes place, as we have already shown, with a few trivial and uncertain exceptions. ”

“ Thus, when a gas is absorbed by water, much heat is evolved, and we may suppose the result to be due in part to chemical combination, and in part to the condensation of the gas into a liquid, by which means its latent heat is rendered sensible. On the other hand, when a liquid unites with a solid, or when two solids unite with one another, and the product is a liquid, we have very often the absorption of heat, the heat rendered latent by the dissolution of the solid being more than that generated by combination. ”

“ When the contiguous winds are feeble, but the temperatures are very different and the zones are near the equator, then the position of the mixture can be inverted by condensation, since the influence of difference of pressure becomes predominant. At the equator, whatever may be the difference of temperature, a mixture that is accompanied by condensation always rises if the surface of separation is stable. The condensation increases by the expansion, each zone of mixture being an outburst of ascending cumuli. ”

“ Since the aqueous material is always working precisely at its evaporation temperature, this gain in energy of position and consequent loss of latent energy will be accompanied by an equivalent condensation and conversion of the rising vapour into the liquid form. ”

“ This partial condensation is sometimes visible. When the sun shines clearly, at the east or west, through a small opening in the clouds, the condensing vapor is shown by the streaks of sunlight, just as the fine particles of dust are seen in a dark room, when a few rays of sunlight are admitted through a small aperture. This phenomenon is often observed, and it is said of it—“It’s a going to rain; the sun is drawing water.” ”

“ By the process of evaporation, it is transformed into vapor and is carried over the land by currents of air. As it comes into contact with colder currents, condensation ensues and then precipitation, and our raindrop descends to earth once more. ”

“ Cold air, as well as cold ground, condenses the vapor of warmer air. If you watch what goes on along the course of a river, you will often see examples of this kind of condensation. The ground on either side of the river parts with its heat after sundown sooner than the river itself does, and consequently cools the air above it more than the air above the river is cooled. ”