vi. J MARLS. 6 1 



general character of the soil to which it is added. 

 The lime thus added to the land is in the form of 

 carbonate ; it has none of the energy which distin- 

 guished the quick-lime. In some respects it is 

 capable of assisting the fertility of the land as much 

 as the caustic lime; but it requires more time to 

 accomplish its work. It neutralizes any organic acids 

 in the soil, it contributes a supply of lime as plant 

 food in a manner very similar to burnt lime, and it 

 exerts a powerful influence upon the mechanical 

 condition of the soil, although in a somewhat different 

 manner. 



1 1 8. Another large and valuable source of lime is 

 found in a class of earths, known as marls. These 

 always contain some carbonate of lime ; but the quan- 

 tity varies greatly, some having about 6 or 8 per cent., 

 whilst others contain 80 per cent, of carbonate of lime. 

 They differ also in the proportion of phosphate of 

 lime and of potash which they contain. The quantity 

 and composition of the silicates present also vary very 

 greatly. The value of a marl is therefore entirely 

 regulated by the fertilizing matter which it contains, 

 and the mechanical influence which it is capable of 

 exerting upon the soil to which it is applied. For a 

 long time these differences in the composition of marls 

 were not understood. Farmers found by the use of 

 marls, that one was better worth drawing ten miles 

 than another marl was worth drawing one mile, and 

 they persevered in their practice. When, however, 

 by the aid of chemistry the difference in their com- 

 position was shown, then the mystery was fully 

 explained, and the evidence of practice fully justified. 



119. The question is frequently asked, How am I 

 to know whether lime should be used as caustic lime, 

 or as carbonate of lime ? To determine this question 

 you must decide upon the result you wish to obtain. 

 If the land should be a sandy soil, with very little 

 organic matter in it, and very weak powers of vege- 



