246 CONSIDERATION OF CARBON COMPOUNDS. 



not necessary that this removal of atoms should be practically 

 accomplished in order to call a group of atoms a residue, but it 

 is sufficient to prove that the unsaturated group of atoms exists 

 as such in a number of compounds, and that it can be trans- 

 ferred from one compound into another without suffering de- 

 composition. 



Residues exist in organic and inorganic compounds ; an in- 

 organic residue spoken of heretofore is the water residue or 

 hydroxyl, HO, obtained by removal of one atom of hydrogen 

 from one molecule of water. Hydroxyl does not exist in the sepa- 

 rate state, but it exists in hydrogen dioxide, H 2 2 , or HO OH, 

 and is also a constituent of the various hydrates or hydroxides, 

 as, for instance, of KHO, Ca2HO, Fe 2 6HO, etc. 



If one atom of hydrogen be removed from the saturated 

 hydrocarbon 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 hydrate, CH 3 HO. 



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

 residue methylene, CH 2 , is left, capable of forming the com- 

 pounds CH 2 C1 2 , CH 2 2HO, etc. 



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

 lent residue CEf is left, capable of combining with three 

 atoms of univalent elements, as in CHC1 3 , or with another triva- 

 lent residue, etc. 



Chains. The expression chain, designates a series of multi- 

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

 held together in such a manner that affinities are left unsatu- 

 rated. For instance : 

 ^ 



_O O , O , O O O , 



are oxygen chains, each one of which has two free affinities 

 which may be saturated, for instance, with the following results: 



H O H, H O O Cl, H O O Cl. 



Hydrogen peroxide. Chloric acid. Perchloric acid. 



In a similar manner, carbon atoms unite, forming chains, as, 

 for instance : 



II III I I I I 



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



II III I I I I 



