CAKI'.OX AND THE JIYI>K< )( 'A KP,< >N> 345- 



C 3 H, , &c. . . do not exist. Those containing the maximum amount of 

 hydrogen will be represented by CH 4 (n=l, 2w + 2 = 4), C 2 H 6 (n='2) f 

 C 3 H 8 (rc=3), C 4 H 10 , lire. This may be termed the law of limits. 

 Placing this in juxtaposition with the law of even numbers, it is easy 

 to perceive that the possible hydrocarbons can be ranged in series, the 

 terms of which may be expressed by the general formuhe C, ( H 2 , (+2 , 

 C,,H 2 ,,, C,,H 2 ,,_,, Arc. . . Those hydrocarbons which belong to any one- 

 of the series expressible by a general formula are said to be homologous 

 with one another. Thus, the hydrocarbons CH 4 , C 2 H G , C 3 H 8 , C 4 H 10 , 

 c. . , are members of the limiting (saturated) homologous series 

 C, ( H. 2 , ( +. 2 . That is, the difference between the members of the series is 

 GEL. 26 ISTotonly the composition, but also the properties, of the mem- 

 bers of a series tend to classification in one group. For instance, the 

 members of the series C,,H 2w+2 are not capable of forming additive 

 compounds, w r hilst those of the series C, ( H. 2 , ( are capable of combining 

 with chlorine, sulphuric anhydride, tire. ; and the members of the 

 C, ( H 2; ,_ G group, belonging to the coal tar series, are acted on by nitric- 

 acid, and have other properties in common. The physical properties, 

 of the members of a given homologous series vary in such a manner, the 

 boiling point generally rises and the internal friction increases as n in- 

 creases 2 " that is, with an increase in the atomic weight ; the specific- 

 gravity also successively changes as n becomes greater. 28 



an unsaturated hydrocarbon, or its derivative, on combining with rX 2 , gives a substance 

 which is saturated or else approaching the limit. The investigations of Frankland with 

 many organo-metallic compounds clearly showed the limit in the case of metallic com- 

 pounds, which we shall constantly refer to later on. 



- tt The conception of homology has been applied by Gerhardt to all organic com- 

 pounds in his classical work, ' Traite de Chimie Organique,' finished in 1855 (4 vols.)> 

 where he divided all organic compounds into fatty and aromatic, which is in principle 

 still adhered to at the present time, although the latter are more often called benzene 

 derivatives, on account of the fact that Kekule, in his beautiful investigations on the 

 structure of aromatic compounds, showed the link which unites them all with the 

 ' nucleus ' benzene, C H . 



27 This is always true for hydrocarbons, but for derivatives of the lower homo- 

 logues it is sometimes different ; for instance, in the series of saturated alcohols. 

 0,,^,,,+! (OH), when n = 0, we obtain water, H(OH), which boils at 100, and whose specific 

 gravity at 15 = 0".)'.M)-2 ; when w = l, wood spirit CH 5 (OH), which boils at 66, and at 15 

 ha- a specific gravity = (V7964 ; when w = 2, ordinary alcohol C 2 H.-,(OH), boiling at 78, 

 specific gravity at 15 = 0'7986, and with further increase of CH. 2 the specific gravity in- 

 creases. For the glycols C n H. 2 (OH) 2 the phenomenon of a similar kind is still more 

 striking ; at first the temperature of the boiling pointand the density increase, and then 

 for higher (more complex) members of the series diminish. The reason for this pheno- 

 menon, it is evident, must be sought for in the influence and properties of water, and 

 that strong affinity which, acting between hydrogen and oxygen, determines many of 

 the exceptional properties of water (Chap. I.). 



28 As, for example, in the saturated series of hydrocarbons C,,Ho n +.o, the lowest 

 member ( = 0) must be taken as hydrogen H.,,, a gas which (t.c. below 190) is 

 liquefied with great difficulty, and when in a liquid state has doubtless a very small 



