Matter and Energy. 21 



Chemical combination, as a form of kinetic energy, is of 

 course always attended by the evolution of heat, or other 

 manifestations of energy resolvable ultimately into heat. 



SECTION III. CLASSIFICATION OF CHEMICAL 

 COMPOUNDS. 



The degree of chemical affinity between elements or com- 

 pounds varies with the elements or compounds under con- 

 sideration. For example, the affinity between carbon and 

 oxygen is much greater than that between carbon and 

 hydrogen ; hence the potential energy in the separation of 

 carbon and oxygen atoms is greater in amount than that in 

 the separation of carbon and hydrogen atoms. There is a 

 marked corresponding difference in the firmness of the re- 

 sulting compounds. Indeed, it is possible to arrange the 

 various, almost innumerable, chemical compounds in a long 

 chain or series, beginning at the one end with those that 

 require the expenditure of an enormous amount of energy to 

 bring about their decomposition, and ending with those that 

 can scarcely be got to hold together at all, that indeed require 

 a constant expenditure of energy to make them do so. The 

 former may be known as stable and the latter as unstable 

 compounds. The classification is a useful one, but is one 

 in which, as in most natural classifications, no hard-and fast 

 line of demarcation can be drawn between the two classes. 



Generally speaking, the more complex a compound is, 

 that is to say, the greater the number of atoms entering into 

 its composition, the more unstable it is, and the fewer the 

 number of atoms in a compound the more stable it is ; 

 soda, water, carbonic acid, iron -rust, and such like, are 

 examples of simple compounds, containing from two to five 

 atoms each ; whilst the majority of the unstable compounds 

 contain twenty, fifty, a hundred, or more atoms. Haemo- 

 globin, the red colouring matter of blood, the most com- 

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