454 CARBON COMPOUNDS. 



in fig. 122. Here each of the six carbon atoms is always united to one of its 

 neighbours by one, and to the other by two, bonds, and thus only six bonds remain 

 free. When these are saturated by atoms of hydrogen, we have a molecule of that 

 important compound called benzene. In all the special instances hitherto mentioned 

 the free bonds of the carbon atoms have been satisfied by atoms of hydrogen, and 

 these combinations have all been found actually realized in nature. It is an 

 extremely important property of carbon, as regards the chemistry of vegetable 

 substances, that all the free bonds of its groups of atoms, no matter how numerous 

 these may be, can be satisfied with hydrogen. Whilst other elements can only form 

 a very limited number of hydrogen compounds, we have a practically unlimited 

 quantity of hydrocarbons. But this is not all. These hydrocarbons form the 

 foundations of innumerable other compounds which are produced by the displace- 

 ment, by atoms of other elements, of one or several 

 atoms of hydrogen in each member of the hydrocarbon 

 series. Many of the substances occurring in plants are 

 hydrocarbons in which a part of the hydrogen has 

 been displaced by oxygen; in others the hydrogen is 

 partly replaced by nitrogen; or for the hydrogen may 

 be substituted the so-called compound radicles (groups 

 of atoms which play the part of an element in com- 

 bination), as, for example, cyanogen, hydroxyl, &c. If 

 Fig 122 . the number of compounds in which carbon is com- 



bined with nitrogen is indeed large, the number of 



compounds obtained from them by the partial replacement of the hydrogen by 

 some other element, and known as derivatives of hydrocarbons, becomes almost 

 beyond conception. 



Finally, the astounding variety which one and the same compound can exhibit 

 in its outward appearance, in form, colour, hardness, and transparency, in taste, 

 and in smell, is due to the inexhaustible permutations in its percentage com- 

 position, which is shown by the hydrocarbons as well as by their derivatives. 

 The same phenomenon is here repeated as is observed in pure carbon uncombined 

 with any other element. It is known that carbon appears either amorphous as 

 charcoal, or crystalline as diamond, or as graphite in the latter case, in crystals 

 which belong to another system than those of the diamond, and differing from 

 them in colour, hardness, and specific gravity. It is not easy to imagine a 

 greater contrast as regards physical properties than that shown by these three 

 substances, and yet it is beyond question, that, chemically, they are identical. 

 The same thing happens in some of the compounds of carbon. Dextrin, 

 starch, and cellulose all have, for example, the same percentage composition; 

 each molecule contains six atoms of carbon, ten of hydrogen, and five of oxygen. 

 And yet how different these bodies seem to our senses; how different is their 

 behaviour to heat and light, to various solvents, and to other chemical compounds! 

 We explain this remarkable phenomenon by the way in which the atoms are 



