XXVI CONSTITUTION OF CHEMICAL COMPOUNDS. 



H H 



H.C.H + ci.ci = ii.q.ci + H.CI. 



H H 



/" ? "\ HH 



21 H.C.C1 I + 2Na = H-c-c-H + SNa.Cl, etc. 



V H J H H 



Each one of the compounds obtained in this way, as well 

 as marsh gas itself, may be looked upon as a compound of 

 one or more hydrogen atoms, with a residue or residues of 

 corresponding valence; and each one of these residues can 

 and does play the part of an element. Marsh gas, when 

 divided as above, leaves, as we have seen, the residues 



H H 



H.C.H (methyl), .C.H (methylene), .C.H, and .0., which 

 are respectively mono-, bi-, tri- and tetravalent. Now, in 

 the formation of the hydrocarbons (substances containing 

 only hydrogen and carbon) from marsh gas, these four 

 residues are the "elements," which are employed, and, by 

 careful examination, we see that here an infinite variety 

 presents itself. If we take the hydrocarbon C 4 H 10 = 



HHHH 



H.C.C.C.C.H, we see that it consists of 2CH 3 and 2CH 2 ; 



HHHH 



but these atoms can be arranged in another way, and the 

 composition C 4 H 10 still be retained : 



HHH 



' C* 



H- ' -H 



H-OOOH; 



H- -H 



H 



in this case we have 3CH 3 and 1CH. This principle can 

 be carried out further, showing the possibility of a very 

 large number of compounds of the same composition, but 

 different constitution. This difference in constitution gives 

 rise to a difference in the properties of the compounds. 



In the hydrocarbons we can replace hydrogen by other 

 elements or groups, and thus obtain the other possible 

 compounds. The replacement by monovalent elements re- 

 quires no explanation, as the constitution of the resulting 



