Vaporization, the conversion of the particles of liquids, and in some instances of solids, into a gaseous condition. When the action takes place from the surface, it is called evap' oration; when from the interior of the mass, ebullition. Evaporation takes place at all temperatures with most liquids, but there are some which cease to give off vapor at reduced temperatures; thus if two vessels, one containing oil of vitriol and the other a solution of chloride of barium, are placed together under the exhausted receiver of an air pump, no precipitate of barium will be produced in the latter vessel, showing that no sulphuric acid has evaporated. Most solids before passing into vapor become liquid; but several, as iodine and camphor, pass immediately from the solid to the vaporous condition. Ebullition differs remarkably from evaporation, and with a few exceptions always takes place in a given liquid, subjected to the same external pressure, at the same temperature. (See Boiling Point, and Evaporation.) The amount of vaporization which will take place from any liquid will depend upon its molecular composition, its temperature, and the space in which it is confined. The laws of vaporization are conveniently studied by observing the formation of vapor in the upper end of a barometer tube.
A glass tube from 36 to 40 in. long and half or three quarters of an inch in diameter, closed at one end and filled with mercury, may be inverted in a reservoir of that liquid, when the column will fall until it is balanced by the pressure of the atmosphere, and a vacuum will exist in the upper part of the tube. If now a drop or two of sulphuric ether is passed into its mouth, it will on arriving at the surface of the mercurial column meet with no resistance to the vaporization of its particles, and it will consequently fill the previous vacuum, and produce by its expansion a pressure on the surface of the mercurial column, which will therefore fall to such an extent that its weight, added to that of the force of expansion of the ether vapor, will balance the pressure of the atmosphere. The introduction of one or two drops more will cause a further fall of the mercurial column, until after several repetitions the depression of the column will cease, but simultaneously there will be a collection of fluid ether at the surface of the mercury and on the sides of the tube, and no drop of ether subsequently introduced will suffer vaporization unless the temperature is increased.
If, however, the tube is warmed, the tension of the ether vapor will be increased; and if the process is continued a point will be reached where all the mercury will be expelled from the tube, and the latter will be filled with the vapor of the ether, the tension of which will then be equal to that of the atmosphere. The temperature at which this takes place is the boiling point of the ether. The ether at any stage of the experiment will boil at the temperature it may have attained when the pressure upon its surface is equal to that which it sustains under the circumstances, viz., the difference between the column in a barometer and that in the experimental tube. For instance, if the column in the latter is 22 in. while the barometer marks 30 in., then the ether, at the temperature to which its vapor has been raised, will boil when sustaining an atmospheric pressure equal to a mercurial column 8 in. high. If the experiment is varied by depressing the tube in the reservoir, it will be seen that, although at the beginning nothing but the invisible vapor of ether occupied the upper part of the tube, when the degree of depression is sufficient, liquid ether will make its appearance.
Until this point is reached the mercury will continue to descend to a lower level during the depression of the tube; but as soon as the condensation of vapor into liquid ether begins, the mercury will remain at the same level until all the vapor is condensed, and the upper portion of the tube is filled with liquid ether. When the tube is raised, the mercury will continue to stand at the same level as long as there is any liquid ether in the upper part; but as soon as it all vaporizes, the mercury will begin to rise, and continue to do so with the elevation of the tube. When such a quantity of vapor is contained in a space that it cannot be condensed, the temperature remaining the same, without a portion passing into the liquid state, it is said to be saturated for that temperature. A certain volume of saturated vapor, therefore, when it is heated, ceases to be saturated, and when exerting a pressure equal to that of the atmosphere does not represent the boiling point of the liquid; this it does only at the points of saturation and of equal pressure. (The boiling points of alcohol, ether, water, and other fluids are given in Boiling Point, vol. ii., pp. 793 and 796.) The condition of saturation of a vapor is therefore that of its maximum tension at the same temperature, since it cannot be compressed with partial condensation into a liquid.
The more a vapor is removed from its saturation point, either by expansion or by increase of temperature, the more nearly does it resemble in physical properties a permanent gas; and it has therefore been concluded that the so-called permanent gases are only vapors which exist at ordinary temperatures far above their points of saturation, and that by simultaneously lowering their temperature and subjecting them to pressure they could be reduced to a point below that of saturation, and therefore that they could be partially liquefied. In many instances experiment has verified the correctness of this conclusion, as in the liquefaction of nitrous oxide, carbonic anhydride, ammonia, and several other gases. (See Heat, vol. viii., p. 578.) The passage of a vapor into a vessel containing a permanent gas, or into the atmosphere, follows the laws of the diffusion of gases (see Gas, vol. vii., p. 633), and it is found that the combined tension of the gas and the vapor is nearly equal to the sum of the separate tensions of the gas and vapor, when contained in the same space at the same temperature.
This is indicated in the increased tension of the atmosphere when it contains a more than usual quantity of invisible watery vapor, and in its diminished tension when such invisible vapor begins to condense and form clouds, as evidenced in the fall of the barometer. The vaporization of liquids under circumstances in which their surfaces are not in contact with the air or any gaseous space, as when water globules are enveloped in oil, presents many remarkable phenomena, some of which are described and explained in the article Boiling Point, as also the effect which substances in solution have upon the vaporization of liquids, as well as the relation of the chemical constitution of a liquid to its boiling point, and an outline of the mode of measurement of the tension of vapors. (See also Expansion, and Steam.) - The following table, from Miller's "Chemical Physics," gives some of the results of Eegnault's experiments upon the tension of vapors of several liquids at equal temperatures. The tension is measured by the height of a column of mercury which each vapor will support at a given temperature, the degrees being given on both the centigrade and Fahrenheit scales.
TEMPEEATUBE, cleg. C.
Bisulphide of carbon.
Oil of turpentine.
The vaporization of liquids under greatly increased pressure, as when they are heated in confined spaces, also exhibits remarkable phenomena. Alcohol, when heated in a space a little more than twice its volume to 404-6° F., expands to twice its original volume and suddenly becomes converted into vapor. A glass tube one third filled with water becomes opaque when highly heated, and bursts after a few seconds. The opacity of the tube is due to the chemical action of the water. If the space occupied by the water is one fourth of that of the whole tube, the liquid will be converted into vapor at about 772° F. When chloride of ethyle is heated in a very thick sealed glass tube, the upper surface becomes indistinct at about 338°, and is replaced by an ill defined nebulous zone. As the temperature rises the zone increases in breadth, and becomes more transparent, until finally the tube appears as if empty. Ether becomes completely vaporized in a space three times its volume at about 375°. The passage of a liquid into vapor is attended with a remarkable disappearance of heat, which is converted into another form of energy by which the particles are kept asunder.
In ordinary language, such heat is said to become latent. (See Heat).