CONSTITUTION OF ORGANIC COMPOUNDS. 449 



Kadicals exist in organic and inorganic compounds ; an inorganic 

 radical spoken of heretofore is the water residue or hydroxyl, OH, 

 obtained by removal of one atom of hydrogen from one molecule of 

 water. Hydroxyl does not exist in the separate state, but it exists in 

 hydrogen dioxide, H 2 O 2 , or HO OH, and is also a constituent of the 

 various hydroxides, as, for instance, of KOH, Ca(OH) 2 , Fe(OH) 3 , etc. 



If one atom of hydrogen be removed from the saturated hydro- 

 carbon methane, CH 4 , the univalent residue methyl, CH 3 , is left, 

 which is capable of combining with univalent elements, as in methyl 

 chloride, CH 3 C1, or, with univalent residues, as in methyl hydroxide, 

 CH 3 OH. 



If two atoms of hydrogen be removed from CH 4 , the bivalent resi- 

 due methylene, CH 2 , is left, capable of forming the compounds 

 CH 2 C1 2 , CH 2 (OH) 2 , etc. 



If three atoms of hydrogen be removed from CH 4 , the trivalent 

 residue CH is left, capable of combining with three atoms of univa- 

 lent elements, as in CHC1 3 , or with another trivalent radical, etc. 



Chains. The expression, chain, designates a series of multivalent 

 atoms (generally, but not necessarily, of the same element), held 

 together by one or more affinities. While such linkage of atoms into 

 chains occurs with a number of elements, it appears that silicon and 

 carbon have a greater tendency to form chains than other elements. 



The linkage of carbon atoms may be represented thus : 



II III I I I I 



_C C , C C C , C C C C , etc. 



II III till 



The above carbon chains have 6, 8, and 10 available affinities, 

 respectively, which may be saturated by the greatest variety of atoms 

 or radicals. The chain combination of carbon, above indicated by the 

 first three members of a series, may, as far as is known, be continued 

 indefinitely. This fact, in connection with the possibility of saturating 

 the other affinities with various atoms or radicals, indicates the almost 

 unlimited number of possible combinations to be formed in this way. 

 In fact, the existence of such an enormous number of carbon com- 

 pounds is greatly due to the property of carbon to form these chains. 



It is not always the case that the atoms when forming a chain are 

 united by one affinity only, as above, but they may be united by two 

 or three affinities, as indicated by the compounds C 2 H 4 and C 2 H 2 , the 

 graphic formulas of which may be represented by 



H \ / H 



