LIME NATURALLY SINKS INTO THE SOIL. 397 



Midland counties of England is said also to last for 30 years, and the 

 same period is assigned to the sensible effect of the ordinary doses of 

 lime-sand in Ireland, and of shell-sands and marls in several parts of 

 France. 



The effect of the lime lessens gradually, and though at the end of an 

 assignable number of years it becomes almost insensible, yet it does not 

 altogether cease till a much ater period. This period is in some cases 

 so protracted that intelligenj: practical men are in many districts to be 

 met with who believe — that certain grass lands would never forget a 

 good dose of lime (p. 391, note). 



§ 19. Of the sinking of lime into the soil. 



One of the causes of this gradual diminution of the action of lime is to 

 be found in the singular property it possesses of slowly sinking into the 

 land, until it almost entirely disappears from the surface soil. It has 

 been long familiar to practical men, that when grass lands, which have 

 been limed on the sward, are after a time broken up, a white layer or 

 band of lime is seen at a greater or less depth beneath the surface, but 

 lodging, generally, where it has attained its greatest depth between the 

 upper, loose and fertile, and the lower, more or less impervious and un- 

 productive soil. In arable lands the action of the plough counteracts 

 this tendency in some measure, bringing up the lime again from be- 

 neath, and keeping it mixed with the surface mould. Yet, through 

 ploughed land it sinks at length, especially where the ploughing is 

 shallow, and even the industry of the gardener can scarcely prevent it 

 from descending beyond the reach of his spade. 



The chief cause of this sinking is to be found in the extreme minute- 

 ness of the particles into which slaked lime naturally falls. If a por- 

 tion of slaked lime be mixed with water it forms a milky mixture, in 

 which some lime is dissolved, but much more is held in suspension in 

 an extremely divided state. When this milk is allowed to stand undis- 

 turbed, the fine particles subside very slowly, and are easily again dis- 

 turbed, but if thrown upon a filter they are arrested immediately, and 

 the lime-water passes through clear. Suppose these fine particles to 

 be mixed with the soil, and the rain to fall upon them, it will carry 

 them downwards through the pores of the soil till the close subsoil acts 

 the part of a filter, and arrests them. This tendency to be washed 

 flown is common not only to lime, but to all minutely divided earthy 

 rnaUer of a sufficiently iricoherent nature. Hence the formation of that 

 more or less impervious layer of finely divided matter which so often 

 form.=? the subsoil beneath free and open surface soils. And that hme 

 ciliould appear alone or chiefly to sink on any cultivated field, may arise 

 from this circumstance — that the continued action of the rains had long 

 before carried downwards the finer incoherent particles of other kinds 

 which existed naturally in the soil, and therefore could find little else 

 but the lime on which this action could be exercised. 



This explanation is satisfactory enough in the case of light and open 

 soils, which are full of pores, but it appears less so in regard to stift 

 clays and to loamy soila which are not only close and apparently void 

 of pores, but seem then.selves to consist of particles in a sufficiently 

 niinute state of divisi-^n to admit of their being carried down by the 



