52 HOW CROPS GROW. 



of the two compounds of carbon with oxygen is given be- 

 low according to both methods. 



Atomic. Per cent. Atomic. Per- cent. 



Carbon, (C,) 13 42.86 (C) 13 37.37 



Oxygen, (O,) 16 57.14 (O 2 ) 33 73.73 



Carbonic oxide, (C O,) 38 100.00 Carbonic acid, (C O a> ) 44 100.00 



The conversion of one of tbese statements into the other is a case of 

 simple rule of three, which is illustrated in the following calculation of 

 the centesimal composition of water from its atomic formula. 



Water, H 2 O, has the molecular weight 18, i. c., 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.: 

 H 2 O Water H Hydrogen 



18 : 100 : : 2 : per cent sought ( = 11.11 + ) 



H 2 O 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 by 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 ORGANIC 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 the elements carbon, 

 oxygen, hydrogen, nitrogen, sulphur, 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 cnemists hun- 

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

 ters, etc. Almost every plant contains some organic body 



