CHEMISTRY OF THE NINETEENTH CENTURY. 631 



There exists another set of hydro-carbons, distinguished from the 

 foregoing and others by their own special chemical properties. Like 

 the former, there are gases, liquids, and solids. These have all one 

 and the same per-centage composition, and therefore by analysis alone 

 they would be indistinguishable ; but by the analytical results taken 

 in conjunction with Avogadro's hypothesis (page 613) we are enabled 

 to assign formulae of these also : 



It will readily be seen that the formula of each compound in both 

 the series differs from that below it by having CH 2 less ; so that the 

 first series may be expressed by this general formula, C n H an + a ; and the 

 second series by C M H an . There are other series of hydro-carbons, and 

 the general formulae of these themselves constitute a still more general 

 series. Thus : 



(a) C w H 2W + 2 . 



w 



(d) 



w 



_ 



C w H aw _ 4 . 

 CH 2M _ 6 , etc. 



The law of this series will be sufficiently obvious. These regular 

 mathematical progressions are very striking, and induced chemists to 

 seek for some theoretical explanation. This is found by attributing 

 to each carbon atom the power of fixing one hydrogen atom at each 

 Q{ four points, and of linking with other carbon atoms also at similar 

 points of attraction. Each carbon atom may roughly be imagined to 

 ha.vef0ttr hooks or bonds, by each of which it may link on to a hook 

 of another carbon atom, or may fix one atom of hydrogen. We shall 

 now graphically represent the atomic constitution on this hypothesis, 

 the four-carbon member of each of the series (a\ (b\ (<:), (d\ and (*), 

 and leave the reader to follow out the like relations in other cases. 

 The black circles are supposed to represent carbon atoms, and the 

 open circles hydrogen atoms, the four bonds of each carbon atom 

 being represented by the four straight lines attached to each circle. 



