338 Dv Davy's Agricultural Discourse. 



Another peculiarity of ahnniiie requires notice, in connection 

 with thorough-flraining, to wit, its power of contracting in drying. 

 No earth absorbs so much water, — whether chemically or hygrosco- 

 pically,* no one retains it so powerfully, or contracts so much in 

 losing it. There are before you precipitates, dried, of alumine, of 

 carbonate of lime, and of carbonate of magnesia. How great is the 

 diif'erencc in their appearance ! that of the alumine is fissured in 

 every direction ; that of the carbonate of magnesia exhibts only a 

 very few fissures; whilst the carbonate of lime has a smooth un- 

 broken surface, indicating no contraction. 



The two peculiar properties of alumine adverted to, and which 

 are also properties of clays, chiefly depending on the presence of 

 alumine, viz., being impermeable to water when expanded by it, — 

 that is, when containing a certain quantity without a free outlet, 

 such as a drain affords ; and being liable to contract, and become 

 fissured and so permeable, on losing water, such as is drawn off by 

 a drain ; these two properties may be considered fundamental ones 

 in connexion with thorough-draining, — the first giving rise to the 

 necessity for the operation, — the second rendering it practicable. 

 In the first instance, it must be supposed, or taken for granted, 

 that the clay is not so compact or condensed by pressure as to allow 

 no passage to water, even with a free outlet, which is a quality, as 

 already remarked of the purest clays. 



For this, the deep and thorough mode of draining, to be most 

 efliciont, should by followed by subsoil-ploughing, Avhich breaks up 

 the clay to a certain depth, and renders it more pervious to water 

 and the access of air, without bringing any of the subsoil to the sur- 

 face. The effect of subsoil-ploughino-, it may be remarked, is v.'ell 

 illustrated, by taking a piece of stiff clay and breaking it up, when 

 it will be found to be readily permeable by water; and again, 

 when the water has been drained from it, compressing it as a plastic 

 mass, when it will recover more or less its impermeability, accord- 



* Wet alumine, from which water had ceased to drop, compared with wet 

 carbonate of lime and fine silicious sand, from which water poured on them in 

 a filter of bibulous jiaper, had also ceased to drop, lost, I find, in drying, 22 

 per cent, more water than the carbonate of lime, and 36 per cent, more than 

 the silicious sand : thus, the alumine lost GO'S per cent. ; cax'bonate of lime 

 38'7 ; and the fine silicious sand 2-t'4 per cent., — at a temperature of air of 

 about SO" Pahr., and when there was a difference of about 10^ between the 

 moistened bulb and dry bulb thermometer. Farther, it may be mentioned, 

 that the alumine kept over strong sulphuric acid, lost 10 per cent, more of 

 water, whilst the carbonate of lime lost only four-tenths, and the sand only 

 two per cent. The carbonate of lime and silicious saud kept over water, 

 shewed no appreciable hygroscopic power ; their weight was not sensibly in- 

 creased ; the alumine similarly placed, after having been similarly dried in 

 the air, acquired water to the extent of 86 per cent. The avidity with which 

 alumine sucks in water is remarkable; it is indicated by the loud crackling 

 noise attending it. I would ask, may not some of the subterraneous sounds 

 not unfrequent in certain clay districts, especially in climates subject to long 

 drouglits, followed by heavy rains, be owing to this cause ? 



