214 CONSERVATIVE ORGANS. [LECT. V. 



sandy matters ; but lime is sometimes, also, found 

 in combination with sulphuric acid, forming a 

 substance called gypsum ; and more rarely with 

 phosphoric acid. When too much calcareous mat- 

 ter is contained in a soil, it is unfertile, owing to 

 its absorbing moisture, and consequently remain- 

 ing too dry. But the case is different when the 

 calcareous matter is mixed with silica, for then the 

 moisture absorbed remains in a free state, and not 

 so united with the chalky matter as to disappear 

 and be useless to plants. But the absorbing pro- 

 perties of all calcareous soils are not alike ; and a 

 great difference depends on the degree of com- 

 minution of the calcareous matter. Thus, 100 

 parts of calcareous sand retain, according to Pro- 

 fessor Schiibler's experiments, 29 parts only of 

 water, whilst 100 parts of the same matter in 

 the state of fine powder retain 85 per cent. In 

 the first case, when calcareous earth and silica 

 predominate in an arable field, they produce a 

 hot and dry soil; when in the second, a moist and 

 cold soil. 



2. ARGILLACEOUS EARTH comprehends clay, 

 which is generally mixed with siliceous sand and 

 mineral substances, and is very retentive of mois- 

 ture. 



3. SILICEOUS EARTH is almost entirely com- 

 posed of sand. The water passes so readily- 

 through it, that very little is retained for the pur- 



