150 ON THE FOOD OF PLANTS. . 



effect with the experiments, cited by Mr. Kirwan, 

 of Dr. Priestley and M. Ruckett. And as to 

 any loss arising by the evolution of any other 

 gases, they are less likely to occur from dung 

 in this situation than when buried ; for carbu- 

 refted hydrogene gas is formed in the greatest 

 quantity during the putrefactive fermentation, 

 and when the substances are immersed in, or 

 glutted \nth, water, and excluded from the air 

 and hght ; and in this state they most generally 

 are when buried : and this gas, possessing an 

 opposite quality to the carbonic acid gas, in be- 

 ing much lighter than the atmospheric air, will, 

 as it is formed, operate in a reverse manner to 

 the carbonic acid ; it will immediately, on being 

 liberated, penetrate the surface of the soil, mount 

 rapidly into the atmosphere, and pass off with 

 the wind, and be thus lost. And further, it is 

 well kno\vn that animal and vegetable substances 

 decomposed by the cold putrefactive fermenta- 

 tion under the earth, or at the bottoms of stag- 

 nant ponds, are inert and inefficient ; at least, 

 until they are made to undergo some other che- 

 mical change, by calcination, oxydation, &c. ; 

 this is evinced by peats, bogs, &c. 



We may likewise instance the effect of the ac- 

 cumulated animal substances decomposed under 

 the earth in burial grounds, which never exhibit 



21 



