310 CAUSES OF FERTILITY AND STERILITY [chap. 



all-important operation will assume its old prominence 

 in the routine of farming. 



Considerable differences are to be seen in the 

 character of lime made from the various calcium car- 

 bonate rocks burnt for lime in the British Islands ; in 

 the main a distinction may be drawn between the white 

 " fat " limes made from the White Chalk, the Mountain 

 Lime^fone and other comparatively pure deposits of 

 calcium carbonate, and the " thin " grey or stone limes 

 made from less pure and more argillaceous limestones. 

 The "fat" limes are the purer, slake readily and swell 

 considerably in the act, forming afterwards a bulky 

 white powder; the "poor" or "thin" limes slake with 

 comparative difficulty and do not increase much in 

 bulk. The " thin " limes partake somewhat of the 

 nature of a cement, setting after mixture with water, 

 and are more esteemed by builders than the " fat " 

 limes, which harden with extreme slowness and are 

 chiefly employed for plastering and kindred work. 

 Naturally the " fat " limes are preferable from an agri- 

 cultural point of view, both from their purity and the 

 finer condition into which they fall ; unfortunately few 

 of the lime works grind the white lime in the ordinary 

 course of trade, as they do the builders' lime. 



The lime made by burning the Magnesian Lime- 

 stone which occurs in Durham, Yorkshire, Derby- 

 shire, and Notts, is disliked by farmers and regarded 

 as injurious rather than beneficial to the land. It 

 contains 50 to 80 per cent, of lime and 4 to 40 per 

 cent, of magnesia, which latter constituent may be 

 the cause of the ill effects. 



The following analyses show the mean composition 

 of several samples of " fat " and " poor " lime, being 

 " white " and " grey " lime respectively, made from the 

 Upper and Lower Chalk of the North Downs : 



