396 TI1E HOME, FARM AND BUSINESS CYCLOPAEDIA. 



insoluble in water and acids, and fuses into a vitreous mass when sub- 

 jected to a white heat. 



Sand quickly dries, and possesses no power to absorb moisture from a 

 damp atmosphere. A cubic foot of sand has been found able to hold 27 

 Ibs. of water as a sponge, i.e., without dripping. Its retentive power to- 

 wards heat renders it useful in the chemical laboratory as a " sand bath," 

 when it is required to keep up a uniform dry heat. Its insoluble, intract- 

 able and simple or elementary characters render it unfit to support plant 

 life. It cannot be said to be in any sense a plant food, but it acts as a 

 divider or opener of the land. It facilitates the percolation of water 

 through the soil ; renders the passage of roots in search of food more 

 easy ; confers a degree of warmth on soils first by drying them, and 

 secondly from its inherent power of retaining warmth ; and renders the 

 soil easy of tillage. All soils contain sand, and its greater or less predom- 

 inance is used as a means of classifying them. 



CLAY. The purest forms are china or porcelain and pipe clays. In the 

 first forms it is found in vast quantities, and becomes the basis of the 

 manufacture of the finest white wares. It is plastic in its character, and 

 to this property it owes its value as a material for making bricks and 

 pottery. The minute particles which form clay have been observed to be 

 crystalline in structure. 



When dry it may be reduced to an impalpable powder. x "When moist- 

 ened it emits the characteristic argillaceous odour, and becomes highly 

 plastic. When subjected to a low red heat it loses its plasticity, and be- 

 comes permanently hard and brittle a fact of great importance not only 

 in the arts but in agriculture. Clay is naturally colder than sand. In 

 chemical language clay is hydrated aluminium silicate, but in nature it 

 is almost invariably associated with potash, soda, lime, ferric oxide, mag- 

 nesia, and carbon dioxide. 



These impurities render clay much more valuable as a constituent of 

 soils than if it were pure. It is in consequence of their presence that a 

 clay soil is very often rich, and that clay ranks among the most import- 

 ant constituents of soils. Pure clay would be as little able to support 

 vegetation as pure sand, but when associated with sand, native or impure 

 clay yields a fertile soil. The special functions it performs in a soil are, 

 first, the maintenance of fertility by the introduction of valuable mineral 

 food constituents ; secondly, clay gives " body " to a soil, by which is 

 meant a certain consistency favourable to the retention of moisture ; and 

 coolness which enables a soil to resist drought. 



