106 SCIENTIFIC AGRICULTURE. 



CLAY. 



Clay is a compound of the t\vo earths, silica and alumina, in a 

 state of chemical union: it usually contains, also, an excess of 

 uncombined silica in the form of sand. Proper clay is formed 

 by nature alone, for no chemical process is known by which 

 silica and alumina can be made to unite so as to form real clay. 

 It is usually colored by some of the oxides of iron, so as to 

 present a bluish, yellow, red or brown hue. 



The two elements of clay are rarely contained in equal 

 quantity ; the silica almost always predominates, sometimes 

 to the amount of 93 per cent. Clay sometimes contains an 

 insoluble carbonate or phosphate of iron, which are both 

 thought to be injurious to vegetation. It sometimes contains 

 also manganese and sulphate of iron, the last of which, unless 

 in a limy soil, is injurious to plants: organic matters are often 

 found in clay, giving it a blackish hue and astringent properties. 

 Clay has been form ad by the decomposition of rocks, such as 

 granite, feldspar, clay slate and argillaceous schist. 



Clay which contains neither iron nor vegetable matter, does 

 not change color by heat: if it contains vegetable matter, it 

 becomes lighter colored by heat; if colored dark by oxide of 

 ircn, it may become lighter by burning, on account of the iron 

 changing its proportion of oxygen. White clay, which does 

 not change color by heat, is nearly or quite pure. When clay 

 is dry, it absorbs water rapidly, becomes tenacious and adhe- 

 sive, so as to retain any form or impression given to it: when 

 saturated with water, it no longer allows that fluid to pass 

 through it: it is from this cause that water stands long on the 

 surface of the ground in swamps having a clay_ subsoil, and 

 this is why we find springs and water veins before we come to 

 solid rock. 



When wet clay is exposed to frost, it is cracked or fissured, 

 and sometimes completely pulverized, by the expansion of the 

 water it contains, during freezing. It retains water with more 



