322 On the Production of Aerolites arid other Solids, 



gen, we must suppose, that leaves of all vegetables inhale a 

 great proportion of carbonic acid gas, which coming in contact 

 with the water of the plant, unites with the hydrogen, forming 

 gums, rcsiu'^, or other mucilaginous matter; which continuing to 

 imbibe a greater proportion of carl^on, becumes concrete, form- 

 ing the wood ; while the oxygen from the carbonic acid gas and 

 water is exhaled by the plant, and greedily absorbed by the 

 atmosphere ; and hence may be the source of the oxygen which 

 is known to appear on the surface^ of leaves when put mider 

 water. 



Since we know that our atmosi)here is of too adhesive a com- 

 position to admit of any foreign gases uniting with it in free 

 space, Ave nmst naturally suppose they are forcibly repelled, 

 and, whether conn,jound or simple, either from their density re- 

 main on the surface of the earth ; or the hydrogen not absorbed 

 bv vegetables iiuiy unite with other bodies for which it has an 

 affinity, dissolving a portion of them, and then, by its levitv, 

 may ascend to the higher regions, carrying with it, in its ascenS 

 other gases also holding bodies in solution. Thus different con- 

 stituents of solid bodies may congregate in the higher regions, 

 where their particles uniting by attraction, by electricity, or by 

 powers luiknown to us, may form precipitable matter of greater 

 density than the atmosphere. 



Considering these circumstances, and the ocular proof which 

 we daily have of solid substances being formed by the imion of 

 gases, why should we question how such as honey dews, gela- 

 tinous showers, &;c. can be produced in the upper regions? And 

 as we have many instances of decomposed animal and vegetable 

 substances being converted into pyrites, undoubtedly from the 

 absorption of sulphur and iron, in many instances, from the at- 

 mosphere, why can we not suppose this composition to t»ke 

 place above, from some substance of an inferior solidity, but per- 

 haps equally capable of attracting particles of matter for the 

 formation of aerolites, which, until having attained a certain 

 degree of density greater than that of the surrounding medium, 

 cannot be precipitated to the earth ? 



As I have before observed, iron is one of the most general 

 fnetallic substances in nature, and I think there is little doubt 

 but it is this which gives the almost universal green colorification 

 to those parts of vegetables which are formed by nature to im- 

 bibe the gases, from the atmosphere, necessary for the constitution 

 of the plant, as I have proved that in those leaves where the 

 colour is greatest there is the largest proportion of iron. This 

 indeed, we may observe by the fallen leaves of trees, &c. that where 

 the colour has been most predominant while living, when dead, 



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