Affinity and Dissected (Structural) Formula. 417 



comparison of the reduced affinity-values of various substances 

 with their chemical properties as summed up in the dissected 

 formulae applied to them, and with their physical properties, and 

 notably their boiling-points. The differences between the re- 

 duced affinity-values for any given pair of substances is a mag- 

 nitude representing the heat that would be generated or absorbed 

 by the performance of an "operation"* on the space filled with 

 the vapour of the first substance, whereby the space becomes 

 filled instead with the vapour of the second body, i, e. when 

 additional (positive or negative) weight is conferred on the space, 

 and a corresponding alteration made in the properties of the 

 gaseous matter filling it. Thus the operation whereby 2 metro- 

 pneums of water-vapour at 100° and 760 millims. are converted 

 into 2 of methylic-alcohol vapour under the same conditions is 

 accompanied by a heat-disturbance of — 57*9 + 53*4, or —4*5; 

 and the operation whereby 2 metropneums of marsh-gas are 

 converted into 2 of acetic-acid vapour is accompanied by a heat- 

 disturbance of -21-7 + 110-0, or +88-3. 



Heat-evolution in such cases is not due to the circumstance 

 that more matter is compressed into a given space, as the heat 

 due to this has been allowed for in each case in the correction h 6 . 



The dissected formulae ordinarily applied to organic bodies 

 may be taken to indicate to some extent the performance of 

 " operations " whereby one substance may be transformed into 

 another. Thus the formulae CH 4 and CH 3 . CO 2 H for marsh- 

 gas and acetic acid, or the formulas C 2 H 5 . OH and C 2 H 5 . . C 2 H 5 

 for ethylic alcohol and ether, represent the hypothetical opera- 

 tions of transformation of 2 metropneums of marsh-gas into 2 

 of acetic-acid vapour, or of 2 of ethylic alcohol into 2 of ether 

 vapour; and hence certain numerical values (viz. the affinity- 

 value differences of the two bodies concerned) are correlative 

 with the differences between two dissected formulas indicating- 

 substances chemically related together. 



27. The increase in affinity-value as homologous series are as- 

 cended (§16) points to the following general rule. Let an ope- 

 ration be performed on a given vapour so as to convert it into a 

 more dense vapour, the chemical relationships between the original 

 and resulting substances being symbolically denoted by the sub- 

 stitution of the group of symbols CH 3 for the symbol H in some 

 constituent hydrocarbonous radical (CH 3 , CH 2 , or CH) in the 

 dissected formula of the original substance ; then heat is evolved 

 during the performance of the operation, which may for shortness 

 be spoken of as "hydrocarbonous methylation." A fortiori, if 

 two or more such operations are performed consecutively (as in 



* Brodie, Phil. Trans, 1866. 

 Phil. Mag. S. 4. Vol. 48. No. 320. Dec. 1874. 2 fi 



