422 EXPERIMENTS on- 



It may be afked, ifmudfei2es oxygen gas with theavi- 

 tlity Ihited, how comes It that eudiometrical experiments 

 prove the air over marlhes to be nearly, if not quite, of the 

 lame degree of purity as that of other fituations ? 



At iirft an anfvver to this important quelHon may fceni 

 difficult ; but fome examination of the circumftances at- 

 tending the fituation of marihes, enables us to account for 

 it in a very fulsfadlory manner. It is to be rcm.arked that 

 in my trials with mud, the air was confined under glafs vef- 

 fels over water, confcquently no circumftances from with- 

 out could have any influence on the experiments. The 

 air over marfhy fituations is very different, it poffeffes all 

 the advantages of ventilation, &c. in common with the at- 

 mofphere. Eefides thefe circumftances, a large quantity 

 of oxygen gas is afforded by the living vegetables which 

 furround them in abundance. We may alfo obferve, 

 that frequently large ponds of v/ater are found in their 

 neighbourhood, and that often rivers are at no great dilhince 

 from them : may not therefore a quantity of oxygen gas be 

 difengaged from thefe waters by the a£lion of the fun ? Ex- 

 periments are related by reputable authors, wherein wa- 

 ter has been decompofed by the a6lion ot the fun's rays ; 

 of this more hereafter. 



That the atmofphere of marflies, therefore, differs in 

 certain circumftances from that of other fituations, and that 

 the foil has confidcrable effeit, in altering the air of the at- 

 mofphere, I think, cannot be doubted. Let us therefore 

 endeavour to difcover the particular local caufes vv-hieh give 

 rife to thefe variations. 



I have before hinted that the putrefaftion of the animal 

 and vegetable matters upon the foil of marflies, was the 

 great caufe of the changes obfcrved to exift : for every fpe- 

 cies of foil will not operate in the manner alluded to. 



That the caufe is in the putrefadiion of thefe matters, 

 and that this ftateis abfolutely neceffary to thofe changes, 1 



infer 



