CARBON COMPOUNDS. 453 



annexed or united to them, or free when this is not the case. When, for example, 

 four atoms of hydrogen unite with one atom of carbon (represented graphically in 

 figure 119, with its four bonds of union), its four bonds are thereby saturated, and 

 a molecule known as marsh gas is produced. Apart from its tetravalency, carbon 

 also has this remai'kable property, that its atoms can also combine with 

 each other, and to a much higher degree than the atoms of any other 

 element. Carbon atoms themselves, and not the atoms of other elements, 

 saturate the separate, free bonds of union in such instances, and in this 

 way are produced groups of atoms, each of which behaves like a '^' 

 chemical unit. Suppose that one of the four bonds of an atom of carbon has united 

 with one of the four bonds of a second carbon atom; then a group of atoms 

 like that shown in fig. 120 will be the result. Where the two carbon atoms 

 have become connected their bonds of union are saturated; but in each atom 

 there are still three unsatisfied bonds, and accordingly they can together 

 annex six atoms of another element. The pair of carbon atoms may 

 now be considered as hexavalent, and if they annex six atoms of hy- 

 drogen, a compound is produced which is called ethane. If three atoms 

 of carbon combine together, so that one bond of each is united to a 

 bond of the neighbouring atom, as represented graphically in fig. 121, 

 four bonds are saturated and eight remain free. These free bonds ng. 120. 

 may be satisfied with atoms of other elements, for example, again with 

 hydrogen. Thus a compound arises which contains three atoms of carbon and 

 eight of hydrogen, and which has been called propane. In like manner four, fi.ve, 

 &c. atoms of carbon may enter into combination together, in which case the remain- 

 ing ten, twelve, &c. bonds of union, which remain free, may be saturated with 

 atoms of other elements. If we suppose that all the free bonds are 

 satisfied by hydrogen, we then have a series of hydrocarbons whose 

 successive members differ from their predecessors by the increment of 

 one atom of carbon and two of hydrogen, but which must each be 

 regarded as a chemical unit, i.e. as a chemical individual and as a 

 particular substance with peculiar properties not possessed by the 

 others. 



Parallel with this series of hydrocarbons run two comparable 

 series, whose members respectively contain two and four atoms of pig. 121. 

 hydrogen less than the corresponding members of the main series; 

 and here the carbon atoms, from which the atoms of hydrogen have been removed, 

 must have combined with one another by the bonds thus liberated. 



The view that several atoms of carbon are only grouped in one direction in 

 linear series, and that the neighbouring atoms are only mutually combined by 

 means of one of their four bonds, as shown in the above graphic representations, 

 is not always confirmed. In many instances we are obliged to suppose that the 

 carbon atoms are distributed in sevei-al directions in space, and are combined into a 

 net-work, or grouped in the form of a hexagon, perhaps in the manner illustrated 



