THE VOLATILE PART OF PLANTS. ^7 



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

 Btatements, and 2, centesimal SDatements, or proportions 

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

 modes of expressing composition are very useful for com- 

 paring toother 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 ^er 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 (COj), 44 100.00 



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

 simple rule 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, 1. e., it consists of two 

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

 parts by weight. - 



The arithmetical proportions subj oined serve for the calculation, viz. : 



HjO Water H Hydrogen 



18 ; 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 required per 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 OEGANIC COMPOUIfrDS, OK PROXIMATE 

 PRINCIPLES. 



.We are now prepared to enter upon the study of the 

 organic compounds, which constitute the vegetable struc- 



