138 ON FALLOW. 



alkalies to be sufficient for both), because the former plants do not 

 require any of the silica necessary for the latter. 



It follows from the preceding observations, that the mechanical 

 operations in the field are the simplest and most economical means 

 of rendering accessible to plants the nutritious matters in the soil. 



But, it may be asked, are there no other means besides the 

 mere mechanical operations, of liberating the ingredients of a 

 soil, and of fitting them for reception by the organism of plants ? 

 There are such means, and one of the most simple and efficacious 

 of them is the practice employed in England for the last century, 

 of manuring soils with burnt lime. 



In order to form a proper conception of the action of lime on 

 soils, we must remember the processes employed by chemists 

 to effect the speedy decomposition of a mineral, and to render 

 soluble its ingredients. In order to dissolve finely-pulverized 

 felspar in an acid, it would be necessary to expose it to continued 

 digestion for weeks, or even for months. But when the felspar 

 is mixed with lime, and is exposed to a moderately strong heat, 

 the lime enters into chemical combination with the constituents 

 of the felspar. A part of the alkali (potash) imprisoned in the 

 felspar is now set at liberty, and a simple treatment of the felspar 

 with acid, in the cold, now suffices to dissolve the lime and the 

 other constituents of the mineral. The silica is dissolved by the 

 acid to such an extent, that the whole assumes the consistence oi 

 a transparent jelly. 



Most of the silicates of alumina and alkalies, when mixed 

 with slacked lime and kept in continued contact in a moist state, 

 behave in a similar manner to felspar when heated with lime. 

 When a mixture of common clay, or of pipe-clay, and water, is 

 added to milk of lime, the whole becomes immediately thicker on 

 agitation. When they are left in contact for several months, it 

 is found that the mixture gelatinizes on the addition of an acid — 

 a property which the mixture of clay and water did not possess, 

 or only to a very small degree, before the contact with lime. 

 The clay is broken up by the union of certain of its constituents 

 with lime ; and, what is still more remarkable, most of the alka- 

 lies contained in it are set at liberty. These beautiful observa. 

 tions were first made by Fuchs of Munich ; and they have not 



