NOTES ON THE SOIL, 485 



water, which is very deficient in sand, but this makes it difficult for a 

 plant growing in a clayey soil to absorb enough water unless the soil 

 contains a large amount. In the latter case the soil is badly aerated, 

 and hence for lack of oxygen the roots are unable to develop to any 

 great extent or to penetrate deeply. 



Pure clay consists of kaolin (silicate of alumina), but in ordinary 

 clays the minute particles cemented together consist of quartz, felspar, 

 or even carbonate of lime (in marls), as well as kaolin. The cementing 

 material is a jelly-like (colloidal) form of kaolin, which swells up on 

 being wetted, making the clay sticky and impervious. When pure 

 clay is mixed with water, the muddy liquid produced shows practically 

 no sign of depositing its suspended matter. On adding mineral acids, 

 or salts, or lime-water, the jelly-like clay is clotted (coagulated), and 

 falls to the bottom. Salts of lime are nearly as good as lime-water in 

 causing this coagulation, while alkaline solutions e.g. soda or potash 

 favour the diffusion of the colloidal clay. Lime is added to clay soils 

 to improve their texture ; clay-particles quickly settle when a river 

 flows into the sea. 



Clay soils are very retentive of moisture and the useful products of 

 manures, but are generally too firm, cold, and damp. Such soils are 

 improved by (1) good drainage* (2) additions of gritty sand, lime, ashes, 

 burnt earth, long straw-containing manure and similar substances 

 tending to increase the porosity of the soil, and (3) deep autumn 

 trenching or tillage, involving the land being left in as rough and 

 cloddy a state as possible for the winter, to secure the disintegration 

 of its particles by the action of alternate freezing and thawing of the 

 water with which its interstices are charged. 



(a) Take two similar lamp-glasses, and to one end of each fasten 

 with sealing-wax a piece of perforated zinc, to form a sieve ; in each 

 case place a piece of blotting-paper in the bottom, covering the zinc. 

 Take two equal quantities of dry clay ; place one lot in one vessel as a 

 coarse powder, moisten the other lot, and make it into a paste before 

 pouring it into the other vessel. Fill up each vessel with water, and 

 measure the volume of water which drains through in each case. 

 Another method is to use two funnels, each with a piece of perforated 

 zinc covered with blotting-paper or filter paper. 



(6) Powder dry clay, then stir it up in hot water and pour it into 

 two vessels (or test-tubes). To one vessel add lime-water, let both 

 vessels stand, and note the result. 



(c) Using the same apparatus as in (a), make a thin paste by working 

 up clay or clayey soil with water, and pour an equal quantity of the 

 paste into each vessel. Fill one vessel up with water (distilled or rain- 

 water if possible), the other with lime-water. Note the time required 

 for 50 c.cs. of liquid to percolate through the clay in each case. 



(d) Repeat (c), using (1) hydrochloric acid, (2) common salt, (3) 

 caustic potash, in solution instead of lime-water. 



(e) Pack clay or garden soil into a glass funnel, then pour in some 

 ammonia- water, and place the funnel over a tumbler. The water will, 



