$58 PRINCIPLES OF CHEMISTRY 



expressible by a general formula are said to be homologous with one 

 another. Thus, the hydrocarbons CH 4 , C 2 H 6 , C 3 H 8 , C 4 H, , &c. . . 

 are members of the limiting (saturated) homologous series C n H 2 ,, +2 . 

 That is, the difference between the members of the series is CH 2 . 26 

 Not only the composition but also the properties of the members of 

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

 of the series C H H 2n+2 are not capable of forming additive compounds, 

 whilst those of the series C n H 2n are capable of combining with chlorine, 

 sulphuric anhydride, &c. ; and the members of the C, t H 2n _ 6 group, 

 belonging to the coal tar series, are easily nitrated (give nitro- compounds, 

 Chapter VI,), and have other properties in common. The physical 

 properties of the members of a given homologous series vary in some 

 such manner as this ; the boiling point generally rises and the internal 

 friction increases as n increases 27 that is, with an increase in the 

 relative amount of carbon and the atomic weight ; the specific gravity 

 also regularly changes as n becomes greater. 28 



Many of tKe hydrocarbons met with in nature are the products of 

 organisms, and do not belong to the mineral kingdom. A still greater 

 number are produced artificially. These are formed by what is termed 



?6 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 vbls.)> 

 in which he divided all organic compounds vaio 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 presence in them all of the 'benzene 

 nucleus/ CflH 6 . 



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

 law is sometimes different; for instance, in the series of saturated alcohols, CnHgn+ifOH), 

 when 7i = 0, we obtain water, H(OH), which boils at 10Q, and whose specific gravity at 

 15 = 0-9992; when n = l, wood spirit CH 3 (OH), which boils at 66, and at 15 has a 

 specific gravity = 0'7964; when w = 2, ordinary alcohol, C 2 H 5 (OH), boiling at 78, specific 

 gravity at 15 = 0'7936, and with further increase of CH 2 the specific gravity increases. 

 For the glycols C n H 2n (OH) 2 the phenomenon of a similar kind 'is still more striking : at 

 first the temperature of the boiling point and the density increase, and then for higher 

 (more complex) members of the series diminish. The reason for this phenomenon, 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 (Chapter I.). 



28 As. for example, in the saturated series of hydrocarbons C/iHgn+2, the lowest 

 member (n 0) must be taken as hydrogen H 2 , a gas which (t.c. below 190) is liquefied 

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

 Where n = l, 2, 3, the hydrocarbons CH 4 , C 2 H e , C 3 H 8 are gases, more and more readily 

 liquefiable. The temperature of the absolute boiling point for CH 4 = - 100, and for 

 ethane C 2 H 6 , and in the higher members it rises. The hydrocarbon C 4 H 10 , liquefies at 

 about 0. C 5 H 12 (there are several isomers) boils at from + 9 (Lvoff) to 87, C 6 H 14 

 from 58 to 78, &c. The specific gravities in a liquid state at 15 are : 



C 5 H 12 C e H 14 C 7 H 16 C 10 H 22 C 16 H34 



0-63 0-66 0-70 0'75 Q'85 



