THE VOLATILE PART OF PLANTS. 37 



Compounds.^ — These are two: 1, atomic or molecular 

 statements, and 2, centesimal statements, or proportions 

 in one hundred parts {per cent, p. c, or %). These 

 modes ol expressing composition are very useful for com- 

 paring together different compounds of the same ele- 

 ments, and, while usually the atomic statement answers 

 for substances which are comparatively simple in their 

 composition, the statement per cent is more useful for 

 complex bodies. The composition of the two compounds 

 of carbon with oxygen is given below according to both 

 methods. 



Carbon Monoxide (CO), 28 100.00 Carbon Dioxide (CO2), 44 100.00 



The conversion of one mode of statement into the other is a case of 

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

 of the centesimal composition of water from its molecular formula. 



Water, HjO, 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. : 



H2O Water H Hydrogen 



IB : 100 : : 2 ; per cent sought (=11.11) 



HjO Water O Oxygen 



18 : 100 : : 16 : per cent sought (=88.89) 



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

 tions, and dividing by the first, we obtain the requiredijer cent, viz., of 

 hydrogen, 11.11 ; and of oxygen, 88.89. 



The reader must bear well in mind that chemical affin- 

 ity manifests itself with very different degrees of inten- 

 sity between different bodies, and is variously modified, 

 excited, or annulled, by other natural agencies and forces, 

 especially by heat, light and electricity. 



§4. 



VEGETABLE ORGANIC COMPOUNDS, OR PROXIMATE 

 PRINCIPLES. 



We are now prepared to enter upon the study of the 

 organic cpmpounds, which constitute the vegetable struc- 



