1 



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 tetra valency, carbon 



also has this remarkable 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- 



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



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



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



j four bonds are saturated and eight remain free. These free bonds pig. 120. 



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



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



I eight of hydrogen, and which has been called propane. In like manner four, five, 



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



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



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



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



! successive members diflfer 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 Fig. 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 several directions in space, and are combined into a 

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



