EVAPORATION. 



169 



The hygrometer of Leslie is an instrument by which the hygrometric state 

 of the air is indicated by the rate at which water evaporates. The bulb of an 

 air thermometer is covered with silk or bibulous paper, which is moistened. 

 The moisture evaporating produces cold in the bulb, and immediately affects 

 the thermometer. The rapidity of the evaporation thus indicated depends on 

 the temperature of the air, and the quantity of moisture it contains. This in- 

 strument, however, is a very imperfect indicator of the hygrometric state of the 

 atmosphere. 



The beautiful theory of evaporation, the details of which we have attempted 

 to explain in the present, and in other lectures, and for the principal part of 

 which the world is indebted to the genius of Dalton, affords a full and satis- 

 factory elucidation of innumerable phenomena which present themselves in 

 atmospheric and meteorological effects, and in all the processes of science and 

 art. 



It has been already explained, that when two liquids, such as water and al- 

 cohol, which combine with a weak affinity, are mixed together, their combina- 

 tion is destroyed by the process of vaporization, and each liquid vaporizes at a 

 given temperature, in the same manner that it would do if it were vaporized 

 independently of the other. The process of the distillation of spirits depends 

 on this principle. Let us suppose that a liquid, composed principally of water 

 and alcohol, is placed in a boiler or still, which communicates by a tube with 

 a refrigeratory or cooler, which is capable of condensing into a liquid the vapor 

 which passes from the still through it. If this mixture be raised to a temper- 

 ature nearly as high as that at which the alcohol would boil, a vapor will rise 

 composed of the vapor of water and the vapor of alcohol, mixed mechanically. 

 Now it will be recollected, that the specific gravity or density of the vapor of 

 alcohol at its boiling point, is about three and a half times that of the vapor of 

 water at 212° ; and again, the density of the vapor of water at 212° is double 

 the density of the vapor of water at 180°. Hence it follows, that the density 

 of the vapor of alcohol at its boiling temperature, 180°, will be about seven 

 times the density of the vapor of water at the same temperature. 



Thus in the steam produced from the mixture of equal parts of water and 

 alcohol, we shall have the proportion of alcohol to water in the ratio of 7 to 1 . 

 This, when condensed in the refrigeratory, will give a strong spirit. By re- 

 peating the process of distillation, the mixture may be more and more sepa- 

 rated from the water which it contains. 



If the distillation be conducted under a diminished pressure, or in a vacuum, 

 the liquid will boil at a much lower temperature ; and the portion of aqueous 

 vapor which will be disengaged will be of such a small degree of density as at 

 length to become insensible. 



The principle on which the process of distillation in general, therefore, de- 

 pends, is, that the constituent parts of the mixture boil at different temper- 

 atures ; and that, if the mixture be caused to vaporize by heat, that part of it 

 which boils at the lower temperature will vaporfze in greater quantities than 

 that which boils at the higher. When the vapor is condensed in the refrigera- 

 tory, a new mixture will then be obtained, containing a much greater quantity 

 of that constituent part which boils at the lower temperature ; and, on the other 

 hand, the liquid which remains in the boiler will contain a greater portion of 

 that which boils at the higher temperature. In general, by conducting the pro- 

 cess in vacuo, or Under diminished pressure, this object is more effectually at- 

 tained, because less in proportion of the liquid which boils at the higher pres- 

 sure will be vaporized in the process. 



In some cases it happens that the temperature necessary to boil the liquid 

 under ordinary pressure may be such as to decompose, or otherwise injure, 



