30 HOW CROPS GROW. 



ULTIMATE COMPOSITION' OF VEGETABLE MATTER. 



To convey an idea of the relative proportions in vi'hich 

 these six elements exist in plants, a statement of the 

 ultimate or elementary percentage composition of several 

 kinds of vegetable matter is here subjoined. 



Grain of Straw of Tubers of Grain of Hay of Red 

 Wheat. Wheut. J^otato. Peas, Otover. 



Carbon 46.1 48.4 44.0 46.5 47.4 



Hvdroeen 5.8 5.3 5.8 6.2 5.0 



Oxveen 43.4 38.9 44.7 40.0 37.8 



NltrSgen 2.3 0.4 1.5 4.2 2.1 



Ash, iiicludii}g sulphur 124 70 40 31 77 



and phosphorus ) ' 



100.0 100.0 100.0 100.0 100.0 



Sulphur 0.12 0.14 0.08 0.21 0.18 



Phosphorus 0.30 0.80 0.34, 0.34 0.20 



Oar attention may now be directed to the study of such 

 compounds of these elements as constitute the basis of 

 plants in general ; since a knowledge of them will pre- 

 pare us to consider the remaining elements with a greater 

 degree of interest. 



Previous to this, however, we must, first of all, gain a 

 clear idea of that force — chemical affinity — in virtue of 

 whose action these elements are held in 'their combina- 

 tions and, in order to understand the language of chem- 

 ical science, must know something of the views that now 

 prevail as to the constitution of matter. 



§3. 



CHEMICAL AFBINITT. — THE ATOMIC-MOLECULAR THEORY. 



Chemical Attraction or Affinity is that force or 

 hind of energy which unites or combines two or more sub- 

 stances of unlike character, to a new body different from 

 its ingredients. 



Chemical Combination differs essentially from mere 

 mixture. Thus we may put together in a vessel the two 

 gases, oxygen and hydrogen, and they will remain uncom- 

 bined for an indefinite time, occupying their original vol- 



