24 FIRST PRINCIPLES OF AGRICULTURE. 



and therefore permeable to air, moisture, and warmth. 

 The stony particles of sand are also rapidly heated by 

 the rays of the sun, thus very greatly influencing the 

 temperature of the soil. 



I/ What is Clay? Pure clay contains silica, together 

 with alumina, a compound of aluminum and oxygen. 

 From a "chemical point of view, pure clay would be 

 quite as useless as sand as a source of plant-food, neither 

 silica nor alumina being essential constituents of plants. 

 The properties of clay are almost the reverse of those 

 of sand. Particles of sand do not adhere to each other 

 moist sand pressed firmly in the hand will fall apart 

 immediately the pressure ceases ; particles of clay, on the 

 contrary, readily adhere to each other, and, when moist, 

 can be moulded into any desired shape, which is retained ; 

 on heating, the particles adhere still more strongly, a 

 characteristic taken advantage of by the manufacturers 

 of brick, tile, etc. Sand from its porous nature rapidly 

 loses water; clay from its compact nature retains mois- 

 ture. Sand absorbs heat rapidly, and soon becomes dry ; 

 clay absorbs heat more slowly, and remains cool. Clay, 

 like sand, is, however, a valuable constituent of soils ; its 

 tenacious character enabling it to retain both moisture 

 and the useful constituents applied in manures. 



Lime. The lime of soils usually exists in the form 

 of carbonate of lime, or limestone. Limestone is a valu- 

 able ingredient of soils, not only because it furnishes 

 the important constituent of plants, calcium, but because 

 of its relative ease of decomposition, anH oi its valuable 

 action upon and reaction with other soil constituents ; 

 it aids in the decay of vegetable matter, and in the 



