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,) 12 42.86 (C) 12 = 27,27 
Oxygen, (0,} 16 5714 (Oz) 32 72.738 
Carbonie oxide, (C ,) 28 100.00 Carbonic acid, (C Oz,) 44 100.00 
The conversion of onc of these 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, Hy O, has the molecular weight 18, i.¢., 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 HE Hydroven 
18: 100 aes 2 +: percent sought (—1111+) 
H,O Water oO Oxygen 
18 : 100 24 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, 
8 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 
