CHTMISTRY APPLIED TO AGRICULTURE. 5 



ordinary proportion ; but from the moment that the agita- 

 tion of the winds can mingle it with the atmosphere, it 

 is spread and diffused towards all points, according to cer- 

 tain fixed laws. Unless in some extraordinary cases, 

 which form exceptions to the general rule, carbonic acid 

 exists in the atmosphere at most only in the proportion of 



Carbonic acid is constantly absorbed and decomposed 

 by the leaves of plants. The carbon is appropriated by 

 the plants to their own support, and the oxygen is thrown 

 out into the atmosphere. Carl3onic acid combines with 

 the lime in fresh mortar, and causes it to return to its 

 original state of lime-stone. Under the pressure of the 

 atmosphere, water will hold in solution nearly its own 

 volume of carbonic acid, and be slightly acidulated by it; 

 but under the force of a greater pressure, it will contain 

 a much greater quantity. Water, thus charged, froths 

 like Champagne wine, which owes its effervescence to the 

 carbonic acid produced by the fermentation of the wine 

 in well-corked bottles. In some recent experiments car- 

 bonic acid gas has been reduced by compression to a 

 liquid state. 



4. Water exists in the atmosphere, under the form of an 

 elastic fluid. When it is absorbed by bodies for which it 

 has a strong affinity, such as the calcined muriate of lime, 

 the portion of air from which it is taken, is diminished in 

 weight and volume. This has been proved by the experi- 

 ments of the elder M. de Saussure and of Davy. The 

 quantity of aqueous fluid contained in the air, varies ac- 

 cording to the temperature of the atmosphere, and in- 

 creases in proportion as that is elevated. At 50^ Fahren- 

 heit it forms in volume nearly 3^ of the atmospheric fluid ; 

 and as its density is to that of the atmosphere in the pro- 

 portion of 10 to 15, it constitutes nearly yL. of its weight 

 (Davy). 



The aqueous fluid forms, when the atmospheric tempe- 

 rature is 34° Centigrade =z 93°.20, (Davy says at 100^) 

 ^ of the volume of the air, and ^\ of its weight. 



The elder M. de Saussure, in his beautiful Treatise 

 upon the Hygrometer, has determined the weight of the 

 water contained in a cubic foot of air, at different tempe- 

 ratures, and has prepared the following table of the 

 results. 



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