EVAPORATION. 



175 



ing the closed part, A B, slightly downward, and pouring the ink in at C, held 

 in a slanting position. When the bottle is placed in the upright position, the 

 surface of the ink in the bottle will remain above the surface of the ink in C, 

 because the atmospheric pressure acting in C will balance the weight of the 

 ink in A B, together with the pressure of the air confined in A B. The evap- 

 oration from the surface in A B having saturated the space above it will cease, 

 and the only evaporation which will have a tendency to thicken the ink will be 

 that which takes place at the surface in C ; but this surface being very small, 

 the evaporation will be inconsiderable. In such an ink-bottle ink may remain 

 several months without thickening. 



The reciprocal processes of evaporation and condensation are the means 

 whereby the whole surface of that part of the globe which constitutes land is 

 supplied with the fresh moisture and water necessary to sustain the organiza- 

 tion and to maintain the functions of the animal and vegetable world. Thence 

 sap and juice are supplied to vegetables, and fluids to animals ; rivers and 

 lakes are fed, and carry back to the ocean their waters, after supplying the 

 uses of the living world. 



The extensive surface of the ocean undergoes a never-ceasing process of 

 evaporation, and dismisses into the atmosphere a quantity of pure water pro- 

 portionate to its extent of surface and the temperature of the air above it, and 

 to the state of that air with respect to saturation. This vapor is carried with 

 currents of air through every part of the atmosphere which surrounds the 

 globe. 



When by various meteorological causes the temperature of the air is re- 

 duced, it will frequently happen that it will come below that limit at which the 

 suspended vapor is in a state of saturation. A deposition or condensation will 

 therefore take place, and rain or aqueous clouds will be formed. If the con- 

 densed vapor collect in spherical drops, it will be precipitated, and fall on the 

 surface of tlfe earth in the form of rain ; but, from some unknown cause, it fre- 

 quently happens that, instead of collecting in drops, the condensed vapor is 

 formed into hollow bubbles, enclosing within them a fluid lighter, bulk for bulk, 

 than the atmosphere. These bubbles are also found to have a repulsive influ- 

 ence on each other, like that of bodies similarly electrified. They float, there- 

 fore, in the atmosphere, their mutual repulsion preventing them coalescing so 

 as to form drops. In this state, having by the laws of optics a certain degree 

 of opacity, they become distinctly visible and form clouds. 



The vapor suspended in the air during a hot summer's day is so elevated in 

 its temperature as to be below the point of saturation, and therefore, though the 

 actual quantity suspended be very considerable, yet, while the air is capable 

 of sustaining more, no condensation can take place ; but in the evening, after 

 the sun has departed, the source of heat being withdrawn, the temperature of 

 the air undergoes a great depression, and the quantity of vapor suspended in 

 . the atmosphere, now at a lower temperature, first attains and subsequently 

 ! passes the point of saturation. 



A deposition of moisture then takes place by the condensation of the redun- 

 dant vapor of the atmosphere, and the small particles of moisture which fall on 

 the surface, coalescing by their natural cohesion, form clear, pellucid drops on 

 the surface of the ground, and are known by the name of dew. 



The clouds in which the condensed vesicles of vapor are collected, are affect- 

 ed by an attraction which draws them toward the mountains and highest points 

 of the surface of the earth. Collected there, they undergo a change, by which 

 they form into drops, and are deposited in the form of rain ; and hence, by their 

 natural gravitation, they find their way through the pores and interstices of the 

 earth, and in channels along its surface, forming, in the one case, wells and 



