52 HOW CROPS GEOW. 



of the two compounds of carbon witli oxygen is given Le- 

 low according to both methods. 



Atomic Per cent. Atomic. Per cent. 



Ciirbon, (C) 13 43.86 (C) 13 37.37 



Oxygen, (O,) 16 57.14 (O2) 33 73.73 



Carbonie oxide, (C O,) 38 100.00 Carbonic acid, (C O2,) 44 100.00 



Tbe conversion of one of tliese statements into llie other is a case of 

 simple rule of tlirei;, which is illastratfd in the fulhiwing calculation of 

 the centesimal composition of water from its atomic formula. 



Water, H^ 0, has the molecular weight 18, i. e., it consists of two 

 atoms of hydrogen, or two parts, and one atom of oxygen, or sixteen 

 parts by weight. 

 The arithmetical proportions subjoined serve for the calculation, viz.: 

 H2 O Water H Hydrouen 



18 : 100 : : 3 : per cent sought ( — U.U + ) 



H2 Water O Oxygen 



18 : 100 :: 16 : per cent sought ( — 88.88 + ) 



By multiplying together the second and third terms of these propor- 

 tions, and dividing hy the first, we obtain the required per cent, viz., of 

 hydrogen, 11.11; and of oxygen, 88.88. 



The reader must bear well in mind that chemical affinity 

 manifests itself with very different degrees of intensity 

 between different bodies, and is variously modified, excited, 

 or annulled, by other natural agencies and forces. 



VEGETABLE OEGANIG COMPOUNDS OR PROXIMATE 



ELEMENTS. 



We are now prepared to enter upon the study of the 

 organic compounds, which constitute the vegetable struc- 

 ture, and which are produced from tlie elements carbon, 

 oxygen, hydrogen, nitrogen, sulpliur, and phosphorus, by 

 the united agency of chemical and vital forces. The num- 

 ber of distinct substances found in plants is practically un- 

 limited. There are already well known to chemists hun- 

 dreds of oils, acids, bitter principles, resins, coloring mat- 

 ters, etc. Almost every plant contains some organic body 



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