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THE GENESEE FAEMEE. 841 



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tables and animals die and rot on the surface of the earth, a similar phenomenon is 



witnessed. 



The gases that form the atmosphere are not chemically combined, but diffused in 

 space in obedience to a peculiar law. If one has a tall glass jar that will hold a half 

 gallon, he may pour into it a half pint of mercury, a half pint of water, and a like 

 quantity of oil, and these liquids, having each a diflerent specific gravity, do not mix 

 nor mingle, but preserve two distinct lines of denlarkation, the one between the quick- 

 silver and the water on the one sirle, and the oil and water on the other. 



Carbonic acid gas, oxygen, and hy<lrogen, have each a different specific gravity, but 

 if we fill the same jar one-third full with the first named gas, which is the heaviest, and 

 one-third with oxygen, which is next in weight, and one one-third with hydrogen, which 

 is the lightest, the carbonic acid, although twenty-two times heavier than hydrogen, 

 will rise to its surface, while the hydrogen will pass down through the oxygen, which is 

 sixteen times heavier than itself, to the bottom of the jar. All gases in the atmosphere, 

 or out of it, behave to each other as they would in a vacuum — each spreading indefinitely 

 into space, regardless of the presence of other gases. Watery vapor obeys the same 

 law of diffusion. 



Whi/ it rains, and how it rains, the formation of dew, and the dew-point, are meteo- 

 rological problems and phenomena, which our young readers may be glad to have 

 explained to them. 



A very dry atmosphere has a thirst for water so intense, that it will absorb moisture 

 from ice many degrees below the freezing point. Evaporation never entirely ceases until 

 after the atmosphere is completely saturated. When this takes place, by what natural 

 process is the water precipitated in rain; snow, and dews 1 These results follow an 

 atmospheric law, by which the air has a varying capacity to hold water as vapor, 

 whether invisible or visible. If the atmosphere had the same power to contain aqueous 

 particles at all common temperatures, it could never rain, nor would there ever be a 

 dew. A change of temperature is the cause of rain, hail, snow, dew, mists, and clouds ; 

 and the daily revolutions of the earth on its axis, and its annual circuit round the sun, 

 are the principal causes, not only of a change of temperature and winds, but of summer, 

 autumn, winter, and spring. The humidity of the atmosphere governs in a good degree 

 the evaporation from the leaves of agricultural plants as well as others ; and^ evapora- 

 tion is the measure of circulation from the roots of all trees and all growing crops. 

 Humidity is an important element in agriculture, even when it does not rain. When a 

 current of air saturated with moisture, meets one of a different temperature, and also 

 saturated, a fall of rain is inevitable, because the air at the mean between the two 

 extremes has always a less capacity to hold aqueous vapor than each separate mass of 

 air had before the two were united. Thus : suppose the atmosphere high up near the 

 region of perpetual snow, or the snow-line, saturated with water at 40°, descends 

 to an atmosphere near the earth equally filled with vapor, and having a temperature 

 of 80°. The mean between 40 and 80 is 60°; and if the air at that temperature 

 had the mean capacity between 40 and 80"^ for holding water, it could not rain nor 

 hail, nor snow, nor yield a respectable dew. It is easy to understand that without 

 rain, there could be no land animals or plants ; and therefore the Divine mechanism 

 for dropping fatness from the heavens in showers and dews is the more worthy of study 

 and admiration. 



Heat expands the atmosphere, and increases ifs capacity to take up water and hold it 

 as an invisible vapor. Cold condenses air, and diminishes its power to contain particles 

 of v/ater in any form. The attraction of aggregation brings the separate particles 

 together, often aided by electricity, and the sudden concussion of thunder, and they fall 

 by reason of the attraction of gravitation. It sometimes happens that drops of rain are 

 wholly evaporated before they reach the earth in passing a stratum of dry atmosphere ; 



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