THEORY OF ITS ACTION. 123 



and obtained a slightly increased and nearly equal produce by 

 each application. The experiment, however, is by no means 

 decisive, and deserves repetition. As baryta cannot be supposed 

 to benefit vegetation at least as a food to the plant any action 

 which the sulphate of baryta exercises ought to be ascribed 

 to its sulphuric acid. 



b That sulphuric acid is capable of acting both in reference 

 to the soil and to the plant in two ways at least. If the soil be 

 deficient in this acid, the gypsum will supply it as any other 

 sulphate would do. It will also yield to the growing plant that 

 quantity of sulphur or of sulphuric acid which are necessary to 

 the composition and formation of its several parts. But 

 the acid may likewise exercise a chemical action of a salutary 

 nature upon the constituents of the soil, so as on the whole to 

 make them more available to the plant. And, in the plant 

 itself, besides contributing directly to the production of its parts, 

 it may act as a chemical agent in producing changes upon the 

 ingredients of the sap which may materially aid its growth. 



c That gypsum, as a sulphate of lime, may, in the plant, 

 perform a very different chemical function from either of 

 its constituents. It is so in the human body. Neither lime 

 nor sulphuric acid taken internally produce the same physio- 

 logical effect on the system as gypsum does. Neither would 

 chlorine or sodium produce any of those salutary consequences 

 which common salt, so grateful to all animals, is known to do. 

 So no doubt gypsum, introduced directly into the sap of a 

 living plant, would produce an effect greatly different from 

 those which would follow the introduction of either lime or 

 sulphuric acid. Instead of disputing, therefore, as some are 

 inclined to do, whether it is the one or the other of its constitu- 

 ents to which the special action of gypsum is always owing, it 

 is wise to bear in mind that the compound body, sulphate of 

 lime, like all other compound bodies, is capable of performing 

 functions and acting in reference to other chemical bodies, in a 

 way in which neither of its constituents could act, and that to 

 this mode of action its peculiar virtues may occasionally be owing. 



One of the properties of this compound is, that, in a moist 

 state, it is capable of decomposing carbonate of ammonia, form- 



