Page:The American Cyclopædia (1879) Volume VI.djvu/816

 796 EVAPORATION the elasticity and promotes precipitation. Pressure does not affect the capacity of air to contain vapor ; but evaporation proceeds more slowly by its increase. If it be removed, as when a liquid is placed in an exhausted receiver of an air pump, evaporation goes on with great rapidity. Ether may thus at ordinary tempera- tures be thrown into ebullition. A difference is observed in the tendency of different liquids to pass into vapor; the lower their boiling point, the more rapid is their evaporation; but it is also observed that the vapor thus easily produced is correspondingly less rare, occupy- ing less space than that requiring a greater ex- penditure of heat for its evolution. The den- sity of alcoholic vapor is 2 '5 times greater than that of water. Dalton discovered that the presence of air or any gas impeded evaporation by the resistance its particles opposed to the circulation of the vapor ; but whether any gas were present or not, the same amount of vapor would always be formed at the same tempera- ture. The effect of the air was seen in the longer time required to fill the space with the amount of vapor belonging to the temperature. Vapors have a greater capacity for heat than their particles when condensed into liquid or solid form. In their formation consequently they abstract heat from surrounding bodies, producing an amount of cold corresponding to the rapidity of the process. This principle is applied in the water and wine coolers used in hot countries. The water with which they are filled, and in which the wine bottles are placed, filters through the porous vessels and evapo- rates from their surface, cooling all the con- tents. A similar effect is experienced in the animal body by rapid evaporation. The heat generated by the chemical actions going on within is taken off by the vapor formed 'at the surface. Damp clothes furnish the means for the production of much vapor and conse- quent reduction of temperature, often to an in- jurious extent. The heat abstracted by vapor in its formation is given out on its condensa- tion. In low pressure steam engines it is econo- mized by being transferred in the condensers to the water that is returned to the boilers. Hygroscopes and hygrometers, already referred to, are instruments designed, the first for de- tecting the presence of moisture in the atmos- phere, and the second for determining either the temperature at which the air under obser- vation begins to shed its moisture, called the dew point, or else the temperature of evapora- tion. Either of these and the normal temper- ature of the air being known, the elastic ten- sion of the atmospheric vapor, and the amount of moisture in a given quantity, are approxi- mately ascertained by reference to tables con- structed for this purpose. (See DEW POINT, and HYGROMETRY.) Evaporation is accompa- nied with ebullition when the elastic force pressing upon the surface of a liquid is less than that due to the temperature of this liquid. (See BOILING POINT.) The principles devel- oped by researches in the evaporation of liquids have been applied in a variety of ways to facili- tate and render more economical several prac- tical operations. Sirups are evaporated, as in the refining of sugar, in vacuum pans, or ves- sels in which the atmospheric pressure may be partially taken off by air pumps. A low de- gree of heat only is thus required, and the risk of overheating and burning the sirup is avoid- ed. Extracts are conveniently prepared on the same principle. But when it is desirable to effect the boiling at high temperatures, as for digesting bones and subjects difficult to dissolve, the evaporation is prevented by the vapor be- ing confined, so as to exert its elastic force upon the surface of the fluid. Thus the escape of more steam is checked until, by greater heat, its elastic force is made greater than that upon the surface. By this method the temperature of the water has been raised to more than 400 F. Eapid evaporation has been promoted in salt works and in bleacheries by causing cur- rents of air to blow over the extended surfaces of the liquids, thus constantly 'bringing new portions of dry air to absorb fresh quantities of moisture. As evaporation cannot take place unless the elasticity of the vapor can overcome the superficial tension of the liquid, and as this tension depends to a large extent on the nature of the gas in contact with the free surface of the liquid, it follows that the boiling point must vary with every circumstance. Thus Dufour, having dropped some pure water into a mixture of oils having nearly the same den- sity, was able to raise the temperature to 356 F., and still saw drops of water swimming in the mixture, although the tension of the aque- ous vapor at this temperature is about 147 Ibs. to the square inch. When water falls upon. red-hot surfaces it separates in spherical drops, which dance around upon the metal, apparently without touching it, and thus continue without evaporating much longer than the fluid would if exposed to the same degree of heat under other circumstances. A platinum crucible brought nearly to a white heat may be almost half filled with water introduced drop by drop, which will continue in this state for some minutes without perceptible evaporation. On cooling the crucible, the liquid suddenly be- gins to boil, and discharges a volume of vapor. While in the spheroidal state drops are seen to be supported on an atmosphere of vapor, which prevents their contact with the surface of the metal. Most liquids, except oils which are de- composed by the heat, display the same pheno- mena. Their temperature while in this condi- tion is not only much less than that of the sur- face upon which they rest, but is also below their own boiling point ; and if they are alreadj boiling when dropped upon the heated surface, the temperature falls to a certain point, which appears to be a fixed one for each liquid in this condition. Water remains at 205 ; alcohol, which boils at 173, falls at least 3 ; ether, which boils at 95, falls at least 5. The tern-