THEORY OF VEGETATION. 45 



open air suffer from the same cause and in the 

 same way ; but though lime in large quantities de- 

 stroys vegetation, in small quantities it renders it 

 more vigorous. Its action is of two kinds, me- 

 chanical and chymical ; the first is the mere divis- 

 ion of the soil by an interposition between its parts ; 

 the second, the faculty of rendering soluble vegeta- 

 ble matter, and reducing it to the condition of ter- 

 reau. 



The mechanical agency ascribed to lime belongs 

 also to marl and to ashes, and in an equal degree ; 

 but their chymical operation, though similar, is less.* 



Gypsum is composed of lime and sulphuric acid. 

 Mayer was the first to present to the public a series 

 of experiments upon it in its relation to agriculture. 

 Many chymists have followed him, and a great va- 

 riety of opinion yet exists with regard to its mode 

 of operation. Yvart thinks that the action of gyp- 

 sum is exclusively the effect of the sulphuric acid, 

 which enters into its composition ; and founds this 

 opinion upon the fact that the ashes of turf, which 

 contain sulphate of iron and sulphate of alumina, 

 have the same action upon vegetation as gypsum. 

 Laysterie, observing that plants whose roots were 

 nearest the surface of the soil were most acted 

 upon by plaster, concludes that gypsum takes from 

 the atmosphere the elements of vegetable life, and 

 transmits them directly to plants. Bosc intimates 

 that the septic quality of gypsum (which he takes 

 for granted) best explains its action on vegetation ; 

 but this opinion is subverted by the experiments 

 of Davy, who found that, of two parcels of minced 

 veal, the one mixed with gypsum, the other left 

 by itself, and both exposed to the action of the sun, 

 the latter was the first to exhibit symptoms of pu- 

 trefaction. Davy's own belief on this subject is, 



* Vegetable ashes are lime combined with an earthy saline 

 matter. 



