44 ELEMENTARY AGHICULTUIIAL CHEMISTRY 



quantity of aluminium silicate more hydrated than the rest, 

 to which the tenacity and plasticity of the clay is due. If 

 this constituent be fully swollen with water the clay is 

 sticky and impervious, while if it be shrunken or coagulated 

 the clay becomes more friable and less plastic. Coagulation is 

 easily brought about by the addition of acids or of many salts ; 

 calcium compounds are particularly effective in this respect, 

 and it is to this cause that the improvement in the texture of 

 heavy clay soils produced by the application of lime is due. 

 It can be illustrated by shaking up some pure clay with dis- 

 tilled water ^nd pouring the muddy liquid into two cylinders; 

 if to one some lime-water be added, the clay will be coagulated 

 or flocculated, and will settle to the bottom in a short time, 

 leaving a clear liquid above it, while the other will fail to 

 clarify even after standing a day or two. 



The plasticity of clay is permanently destroyed by he^t, 

 which expels the two molecules of v^ater of hydration, the 

 anhydrous aluminium silicate left (e,^.,in the making of bricks 

 or tiles) not being capable of again combining with water. 



Limestone. — Calcium carbonate is present in a soil in a 

 finely divided state disseminated among the other constituents, 

 but in addition there are often small fragments which are 

 classed with the " sand." The former condition, however, is 

 the one which is of importance. It furnishes plant food by 

 virtue of the calcium, magnesium and phosphoric acid present, 

 but it plays other, perhaps more important, functions. It 

 modifies the plasticity of the clay in the manner characteristic 

 of calcium compounds, and acts as a weak base, for, though it 

 is a true salt, the carbonic acid is so weak an acid that it 

 is readily displaced by stronger acids, which unite with the 

 calcium, and thus lose their acidity. Acids are produced by 

 the decay and fermentation of vegetable matter, and if large 

 quantities of such material are present in a soil there is often 

 a great tendency for the production and accumulation of free 

 acid, and the conditions then become unfavourable for the 

 growth of most useful plants. Such land is often spoken of as 



