429 



propyï-ethev. in this also a inairKcótation must be seen of tlie differ- 

 ences between normal aiul ramified carbon-chains. In the same way 

 the molecular snrface-energy of (lieth;iltartrate appears to be greater 

 than of dlmetJujltartrate. In the series of aromatic hydrocarbons, the 

 curves for pseudociunene and the isomeric mesitylene are situated 

 highest; then follow successively: p-xijhme, toluene, and benzene; in 

 the same [i is greater for o-toluic/ine than i'ov a}iiUne,fov o-nitrotoluene 

 greater than for nitrobenzene; just so for phenetol greater than for 

 anisol, for dimethylaniline greater than for aniline, and for a-picoline 

 greater than for pyridine. The only exception to this rule hitherto 

 found, is preseuted l)y the resorcine-nionomethyletker, which possesses a 

 greater molecular surface-energy than the corresponding dimethyl-ether. 



The substitution by means of }nend)ers o f the aromatic series has an 

 analogousinjluence as by those of the aliphatic series, but it is much more 

 intensive: in the series of the benzoates, the value of f,t for the ethyl-eXher is 

 indeed, greater than for the methyl-^ev'waXixe, but for the corresponding 

 benzyl-eihev it is excessively much greater ; in the same way it is 

 the case with methyl-, and ethylsalicylates and salol, and with methyl- 

 propylcetone on one side, and acetophenone on the other. 



B. Relations of Substitution- derivatives. 



The conclusions, which in this respect can be drawn hitherto, 

 can be summed up shortly in the following rules: 



J. The substitution of H by halogens is accompanied by an inten- 

 sive increase of the molecular surface-energy at the same temperatures; 

 the influence increases evidendy with augmenting atomic loeight of the 

 halogen. 



So IX for chlorotoluene is greater than ïov jluor o toluene^), and here 

 much greater than for toluene itself; for bromobenzene it is greater 

 than for chlorobenzene, and appreciably greater than for benzene; 

 with the m-dichloi'obenzene it is greater than for jiuorobromobenzene, 

 showing that the specific influence of fluorine seems to be less than 

 the difference between bromine and chlorine. In the same way the 

 value for diethylbromomalonate is appreciably greater than ïov dietyl- 

 malonate; for tetrachloromediane just so greater than ïor chloroform. 



2. The substitution of N-atoms for C-atoms, or of that of negative 

 nitrogen containing radicals for a. H-atom, is followed by a relatively 

 great increase of the molecular surface-energy at the .s-ame temperatures. 



^) The relatively small ditreieiices caused by the struclural isomerism of these 

 compomids, is here neglected for llie present; generally the para-substitution seems 

 to be of the highest, the me^a-substiiulion of the smallest inOuence in this respect. 

 We will discuss this pecularity afterwards by considering the results of a special 

 set of measurements. 



