632 HISTORY OF SCIENCE. 



() C 4 H, 



O 



O O 



I I 



>- 00 



o- - 



I I 

 o o 



c == o 



(<) C 4 H 6 - i--, 



I I 



O O 



to C 4 H 2 



The foregoing also illustrates the notion called the valency or atom 

 fixing power of atoms. Each carbon atom has four bonds or points 

 of attachment, or the power of fixing four atoms having each but one 

 bond. Of this last kind are hydrogen, chlorine, potassium, silver, etc., 

 and these are spoken of as monovalent elements ; while oxygen, etc., 

 are divalent ; and so on up to htxavalency. 



There is another subject which may here be illustrated. If we con- 

 sider with a little attention the representation of the atoms of the 

 group C 4 H IO , it will be seen that there is another mode in which the 

 atoms may be linked, namely, thus : 







C-0-O 



? 



o o r H 



^ 4 n 10 



O 

 0--0 



I 

 o 



