AQUEOUS ATMOSPHERIC PHENOMENA. 



461 





CHAPTER XIV. 



AQUEOUS ATMOSPHERIC PHENOMENA. 



N addition to common air, a combination 

 chiefly of the oxygen and nitrogen gases, 

 united in different proportions, the atmo- 

 sphere contains a mass of invisible vapour 

 insinuated between the particles of the 

 gases, and filtering through them, in a 

 manner which may be compared to that 

 of the diffusion of water through a sponge, 

 or visible in the form of fogs and clouds. 

 This vaporous atmosphere is the result of 

 the ever-active agency of heat and elec- 

 tricity, which, by a process of marvellous 

 subtility and energy, evaporates the waters 

 from the surface of the earth, and trans- 

 fers them for a time to an aerial home. 

 The process is entirely untraceable by the eye of man, but its product appears in the 

 clouds that are reared aloft in fantastic shapes, in the mists that occasionally shroud the 

 landscape, the rain and the snow that come down from heaven, the dew glistening in the 

 morning light, and the hoar-frost which adorns the forest with a beauty that throws the 

 results of human artistic skill into insignificance. The words of the sacred writer are 

 philosophically true : " He calleth for the waters of the sea, and poureth them out upon 

 the face of the earth;" but the supply of humidity furnished to different countries varies 

 prodigiously in its amount, and hence other differences as the consequence barrenness 

 here and fertility there a comparative solitude abandoned to the occupancy of the 

 inferior orders of the animal creation, and a land studded with the homes of peasantry, the 

 palaces of nobles, the halls of science, and the marts of commerce. Though we have 

 spoken of the process of evaporation as untraceable by the human eye, yet that refers to 

 the exhaling agency, for sensible evidence is frequently afforded that the ever-operatiiig 

 machinery is actually at work before us, in the visible exhalations we behold at early 

 dawn, and in the calm evening of a summer's day. It is generally the case, however, that 

 the formation of visible vapours takes place in the higher regions of the atmosphere, 

 though at the earth's level, the metamorphosis of its waters into an invisibly vaporous 

 state is constant, and is proceeding as powerfully when no outward si^n of the process 

 appears, and the air is perfectly transparent, as when a misty mantle, of feathery shape and 

 texture, rests upon the lakes and rivers, and lies upon the surface of the valleys. 



It is not merely from the great collections of water in oceans, lakes, and rivers, that 

 evaporation takes place, but from the pasture grounds and forests ; and Leslie makes the 

 remark that even ploughed land will supply as much moisture to the exhaling fluid as 

 an equal sheet of water. But the atmosphere is only capable of receiving a certain 

 quantity of vapour in an invisible state, its capacity depending upon temperature, and 

 being invariable in its extent at the same temperature. When all the interstices of the 

 gaseous fluid are full, it is then said to be at its point of saturation, and any further supply 

 of vapour becomes visible in the form of steam or mist. The lower the temperature, the 

 greater the condensation of the air and the tightness of its particles, so that only a certain 



