OPERATION OF MANURE. 227 



full of the vapour of- ammonia, and standing over 

 mercury, it will be presently noticed that the 

 mercury rises up into the jar, and a considerable 

 quantity of ammonia has disappeared. The char- 

 coal has undergone no change, neither has the 

 mercury ; on what prin- 

 ciples, therefore, are we 

 to account for the disap- 

 pearance of the ammonia? 

 The explanation is to be 

 found in a very peculiar 

 property of gases, allot- 

 ted to them, it may be, 

 for this very purpose we 

 have been speaking of. 

 By virtue of this property, ammonia, like other 

 gases, is capable of becoming condensed on the 

 surface of bodies, or absorbed into their sub- 

 stance ; some possessing a greater aptitude than 

 others for effecting this process. Charcoal, 

 burnt clay, gypsum, and vegetable mould, all 

 possess this property in a high degree. Dr. 

 Daubeny considers that the use of gypsum 

 arises in part from its property of fixing am- 

 monia, and in part from its being itself directly 

 serviceable to certain species of plants, by sup- 

 plying them with a salt which they require for 

 their development. 



Thus endowed, these substances form appro- 



