OF AGRICULTURE. 163 



called a thoroughly practical farmer, who docs not 

 understand them. 



We have thus far examined the raw material of 

 plants. We have looked at each of the elements, sepa- 

 rately, and considered its uses in vegetable growth. 

 We will now consider another division of plants. 



We know that they consist of various substances, 

 such as wood, gum, starch, oil, etc., and on examina- 

 tion, we shall discover that these substances are 

 composed of the various atmospheric and mineral 

 ingredients described in the preceding chapters. They 

 are made- up almost entirely of atmospheric matter, 

 but their ashy parts, though small, are of great 

 importance. 



These compounds may be divided into two classes. 

 The first class is composed of carbon, hydrogen and 

 oxygen. The second class contains the same sub- 

 stance, and nitrogen. 



The "first class that containing no nitrogen 

 comprises the wood, starch, gum, sugar and fatty 

 matter, which constitutes the greater part of all 

 plants ; also, the acids which are found in some 

 fruits, etc. 



Yarious as are all of these things in their character, 

 they are entirely composed of the same ingredients 

 {carbon, hydrogen and oxygen), only slightly differing 

 in proportions. There may be a slight difference in 

 the composition of their ashes, but the vegetable parts 

 derived from the atmosphere, vary so little that they 

 can often be artificially changed from one to the 

 other. As an illustration of this fact, it may be 

 stated that, at the fair of the Mechanics' Institute, 

 Professor Mapcs exhibited samples of excellent sugar 



