228 MANUAL OF AGRICULTUEE. 



nitric acid, H^O + NgO^ = 2 (HNO3). "With hydrogen, it forms 

 ammonia — 1 atom of nitrogen to 3 of hydrogen — NHg . 



Carbon we meet with free and as a solid in three dis- 

 tinct forms, physically different, but possessing in common 

 the same chemical properties. These are : — («.) The precious 

 diamond ; (h.) graphite or plumbago, popularly known as black 

 lead ; and (c.) charcoal. It is found neither as a fluid nor a gas 

 in the free state. It is present in all organised structures. It 

 forms about 50 per cent, of the residue of plant life when the 

 latter is charred, and access of atmospheric air or oxygen pre- 

 vented, for oxidised carbon escapes as a gas. It enters into 

 exceedingly complicated compounds, the consideration of which 

 forms a special branch of chemical science, called organic che- 

 mistry. Combined with oxygen, it forms carbonic acid CO^ — an 

 invisible, ponderous gas. 



Plant life is unable to assimilate these organic elements in 

 their free state, but only when they exist in combination with 

 certain other elements. Such compounds are water, nitric 

 acid, ammonia, and carbonic acid. In the organised structures 

 of plants these compounds are broken up or resolved, and their 

 constituent parts economised in the building up of new organisa- 

 tions, which in their turn are metamorphosed for the structure of 

 animal life. The atmosphere and soil, but chiefly the latter, are 

 the media through which these elements are rendered available for 

 the necessities of plant life. 



Silicon. — I^ext directino- the attention to the inorganic ele- 



O O 



ments, it is to be remarked that silicon, next to oxygen, is the 

 most abundant element in nature. It does not occur free, but 

 as an oxide, SiOg , known as silicic or silica acid. In that state 

 it is nearly pure, under the forms of quartz, flint, and sand. 

 Silica, though in a variable quantity, is always present in the 

 ashes of plants. Chiefly is it plentiful in cereals and grasses. 

 It forms the hard glistening surface of straw and bamboo. In 

 most plants, however, it rarely exceeds 5 or 6 per cent, of the 

 residual ash. 



Aluminiui'ii is a bright lustrous metal of excessive lightness. 

 It does not occur free, but as an oxide — Al.^Og . There is but a 

 slight trace of it in the ashes of plants, although in combination 

 with silica, under the name of silicate of alumina, it forms the 

 basis of the clay of our soil. From an agricultural point of view, 

 it is therefore of importance. 



Potassmm, when with difficulty prepared free, is a light metal 

 of silvery appearance, and so soft as easily to be cut with a knife. 

 Thrown into water, it decomposes it. One atom of potassium 

 replaces one atom of hydrogen, and heat is evolved in sufficient 

 quantity toi gnite the liberated hydrogen. It rapidly absorbs 

 oxygen from the atmosphere, forming the oxide K2O — Kalium 



