62 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,) 12 42.86 (C) 12 27.27 
Oxygen, ©, 16 57.14 (Ox) 82 72.93 
Carbonic oxiie,(C O,) 28 100.00 Carbonic acid, (C Os,) 44 100.00 
The conversion of one of these statements into the other is a case of 
simple rule of three, which is illustrated in the following calculation of 
the centesimal compositior of water from its atomic formula. 
Water, H, O, 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.: 
H, O Water H Hydrogen 
1k een 100 rae ain Rus per cent sought ( = 11.11+) 
H, O Water O Oxygen 
1S eran 100 2c 16: percent 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. 
§ 4. 
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 chemists hun- 
dreds of oils, acids, bitter principles, resins, coloring mat: 
ters, etc. Almost every plant contains some organic body 
