THE ACTION OF GYPSUM IS COMPLEX. 347 



supply is elTected here by the aid of a lime salt. An 

 experiment, made specially for the purpose, showed that 

 the contact of arable earth with the solution of sulphate 

 of lime is attended by an actual substitution of magnesia 

 for lime ; that is to say, a certain quantity of lime is with- 

 drawn from the solution and combines with the earth, 

 whilst tlie liberated sulphuric acid, which was united to 

 the lime, withdraws from the earth an equivalent quan- 

 tity of magnesia. In a litre of g}^3sum water which 

 had been in contact with 300 grammes of earth from a 

 wheat -field, there were found the following quantities of 

 sulphuiic acid, magnesia, and lime : — 



Besides the magnesia, a certain amount of potash also 

 seems to be dissolved out of the earth by aid of the 

 gyiosum. 



Out of 1000 grammes of earth from a wheat-field, 

 there was dissolved by — 



• 3 litres of pure water 3 litres of gypsum water 



Pota.sli . . . 24-3 milligr. 43-6 miUigr. 



These ex|ieriments show that the action of gypsum is 

 veiy complex, and that it ]:)romotes the distribution of 

 both magnesia and potash in the ground. This much 

 is certain, that gypsum exercises a chemical action upon 

 the soil, which extends to any depth of it, and that in 

 consequence of the chemical and mechanical modification 

 of the earth particles of certain nutritive elements be- 

 come accessible to, and available for, the clover plant, 

 which were not so before. 



The cause of the action of a manuriuix accnt is 



