8 CHYMISTRY APPLIED TO AGRICULTURE. 



Independently of those bodies which essentially consti- 

 tute the atmosphere, there are mingled in it the exhala- 

 tions 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 ascen- 

 sion, ceases to act upon them. These exhalations modify 

 the properties of the air, and affect its purity. The oxy- 

 gen and the water of the atmosphere become impregnated 

 with the particles of the exhalations which are deposited 

 with them upon the surfaces of other bodies, where they 

 remain in contact, or enter into combination, with them. 

 The origin and dissemination of many maladies may be 

 traced to this source ; the germ of them is carried 

 through the air by the aqueous fluid. And for the same 

 reason it is, that intermittent fevers are endemic in those 

 situations, where large quantities of animal or vegetable 

 matter are undergoing decomposition, as upon the borders 

 of ponds and marshes ; and that the miasm, which arises 

 from numerous animal remains in a state of decompo- 

 sition, becomes a fruitful source of disease. It is for the 

 same reason also dangerous, under some circumstances, 

 to breathe the evening air ; the aqueous fluid contained in 

 it is loaded with the noxious principles, which the heat 

 of the sun during the day had caused to ascend into the 

 atmosphere. The disagreeable odor, conveyed to us in 

 mists, is owing to the power of the aqueous fluid in trans- 

 mitting the exhalations arising from the earth. 



The manner in which the air conveys to us the per- 

 fume of plants, and the odor which it contracts from the 

 exhalations of bodies in a state of decomposition, indicate 

 clearly its influence in producing maladies, and still more 

 plainly its power of propagating those that are contagious. 



ARTICLE II. 



Of the Imponderable Fluids contained in the Atmosphere. 



Besides the ponderable substances which constitute the 

 atmosphere, and those which are found in it accidentally, 

 it receives some imponderable fluids, of which the effects 

 are less known, but which appear to play an important 

 part in the atmospheric phenomena ; of this number is 

 the electric fluid. 



