22 TROPICAL AGRICULTURE CHAP. 



Compounds Hydrogen is the lightest of all the elements. It combines 

 y rogen. ^^ gjght parts of oxygen to form water ; and in combina- 

 tion with nitrogen it forms ammonia. Hydrogen is also an 

 important part of nitric, hydrochloric, sulphuric and phos- 

 phoric acids. 



Nitrogen, like oxygen and hydrogen, is an invisible gas, 

 and like them also it plays an important part in all life. 

 The air we breathe is composed of one-fifth of oxygen and 

 four-fifths of nitrogen. The two gases are not combined, 

 like oxygen and hydrogen in water, but are both free in the 

 atmosphere in other words they are mixed together 

 mechanically and not combined chemically. 



Carbon is not a gas, but in chemical combination with 



oxygen it forms the important gas, carbonic acid, which has 



Carbon been already alluded to. Carbon makes up the greater part 



bulk of e of the bulk of all plants, and it is taken up by them, not in 



plants. j S p ure state, but in the condition of carbonic acid. The 



plant under the influence of sunlight then decomposes, or 



breaks up the carbonic acid into its two elements, and whilst 



it keeps the solid carbon to build up its tissues, it gives back 



Respiration the oxygen to the atmosphere. This is just the reverse of 



of animals. 



what occurs in the case of animals, for they take m oxygen 

 and breathe out carbonic acid ; and thus, by a wonderfully 

 wise provision of Nature, the poisonous carbonic acid 

 breathed into the air by animals, is taken up by plants in 

 order that they may keep the carbon and restore the health- 

 giving oxygen to the air. 



The Atmosphere. A good deal has been said about the 

 atmosphere and it is so much concerned in the growth of 

 plants, in the formation of soils, and in the processes whereby 

 they may be kept fertile, that it is necessary for the agricul- 

 turist to have a correct knowledge of its nature. We have 

 Composition seen that the atmosphere, or the air we breathe, is composed 

 f a mechanical mixture of the two important gases, oxygen 

 and nitrogen, in the proportion of about one-fifth of oxygen 

 and four-fifths of nitrogen. The oxygen, although in much 

 less quantity, is the more active element, and the nitrogen 



