164 



EVAPORATION. 



stantaneously ; whereas, if the supposed solvent were present, its production 

 would be considerably retarded. Thus it appeared that the solution would 

 proceed with greater facility in the absence of the solvent than in its pres- 

 ence. 



It has been already shown, that liquids dismiss vapor, whether the space 

 above their surface be an actual vacuum, or be filled with air or other gas, 

 and that if such space be confined within certain limits, it will be capable of 

 receiving from the liquids a different quantity of vapor, depending solely on the- 

 temperature of the liquid, and that the quantity which will saturate a given 

 space will be the same, whether that space be a vacuum, or be occupied by 

 atmospheric air, or other aeriform bodies. The difference in the phenomena 

 in the two cases will only consist in the rate at which the saturating vapor is 

 produced from the liquid. In the case of a vacuum, it is produced almost in- 

 stantaneously ; but if air be present, its production is retarded, and a consider- 

 able time may elapse before the space above the liquid is saturated. 



All masses of water placed on the surface of the globe, have above them a 

 mass of atmospheric air, which at all times maintains suspended in it a quantity 

 of aqueous vapor, raised by the process of evaporation from the surfaces of this 

 liquid. If the quantity sustained in the atmosphere be such as to saturate the 

 air, then it is obvious that no further evaporation whatever can take place at 

 the surface of the water. This, however, does not usually occur. Most com- 

 monly the vapor suspended in the atmosphere is insufficient for its saturation ; 

 and in this case evaporation will take place. It is the object of the present 

 lecture to explain the laws which attend this process of evaporation in the 

 open air. 



Dalton, to whose labors we are indebted in this, as in every other part of the 

 theory of vapors, investigated this subject, and may be said to have nearly ex- 

 hausted it. He commenced by determining the circumstances which attend the 

 evaporation of water at high temperatures. In such cases, the tension of the va- 

 por actually suspended in the air would produce an inappreciable effect on the 

 phenomena, because its tension would be inconsiderable, when compared with 

 that of the vapor of water at high temperatures. In this first experiment, there- 

 fore, he regarded the atmosphere as perfectly dry, and considered the phenom- 

 ena to proceed as they would in a receiver subject to the presence and pressure 

 of perfectly dry air. A small vessel, containing boiling water, was suspended 

 from the arm of a balance, and accurately poised. A lamp was placed under 

 it, which maintained it at the boiling point, and its loss of weight in a given 

 time by evaporation was accurately determined. The same experiment was 

 repeated with the same vessel, at various temperatures, from 212 to 138, and 

 the following results were obtained : 



From this table it is apparent that at each temperature between the above 

 limits, the rate of evaporation is proportional to the tension of the vapor. It 

 will easily be conceived, however, that the same law cannot extend to evapora- 

 tion at low temperatures, because, as the temperature of the evaporating 



