ITS USES TO THE HUSBANDMAN. 239 



gen and one of carbon, the latter being found pure in the 

 diamond, and forming the substance of mineral and wood 

 coals. This gas is produced in abundance by fermenta- 

 tion, respiration, and combustion; is absorbed and decom- 

 posed by the leaves of plants, the oxygen being set free, 

 and the carbon being converted into wood, &c. The 

 causes which produce it, sometimes, in confined situa- 

 tions, give it in such excess as to render it prejudicial to 

 animal health ; but the free access of atmospheric air soon 

 restores the equilibrium. It constitutes much of the prop- 

 er food of plants. Thus animals and vegetables are mutu- 

 ally benefited, through the wise provision of the Creator, 

 by their proximity to each other — plants giving off oxygen, 

 necessary to animals — and animals giving off carbonic 

 acid gas, the pabulum of vegetable life. 



Water also exists in the atmosphere, in the form of an 

 elastic fluid. This fluid is found to form, at the tempera- 

 ture of 50° Fahrenheit, about one fiftieth of the volume of 

 the atmosphere, in the driest time in summer, and is in- 

 creased with the increase of temperature — heat accelera- 

 ting the evaporation of moisture from the earth's surface. 

 When the temperature of the air is diminished, the aque- 

 ous fluid is condensed, and appears in the atmosphere in 

 the form of vapor, or clouds, and is copiously deposited, 

 in summer, in the form of dew. This water is retained, 

 principally, in the lower regions of the atmosphere. It 

 is so slightly united with the other elements of the atmo- 

 sphere, that a change of temperature produces a change in 

 its proportions ; whilst nitrogen, oxygen, and carbonic 

 acid preserve, always, nearly the same relative proportions. 



" Independently of those bodies which essentially con- 

 stitute the atmosphere," says Chaptal, "there are mingled 

 in it the exhalations constantly arising from the earth ; 

 these are again disengaged from the air, and precipitated, 

 as soon as the heat, or any other cause which occasioned 

 their ascension, ceases to act upon them. These ex- 

 halations modify the properties of the air, [by the carbonic 

 acid gas, &c. disengaged from animal and vegetable mat- 

 ters in a state of putrefaction,] and affect its purity. The 

 oxygen and the water of the atmosphere become impreg- 



