1894.] The Molecular Surface-energy of the Esters, $c. 165 



Table I continued. 



Critical 

 temperature. 

 Ester. C. Tc. d. 



Ethyl formate 235'4 2-020 4'5 



Ethyl acetate 251-0 2'226 6'7 



Ethyl propionate 272'9 2'240 4'9 



Propyl formate 264'85 2'110 4'85 



Propyl acetate 276'2 2'227 5'0 



It is evident at the first glance that it is the acid radical which 

 determines the value of Tc, for it increases progressively with the pro- 

 gressive increase of its molecular weight. Young has noticed a 

 similar relation to hold with the ratios of absolute temperatures at 

 corresponding pressures to absolute critical temperatures, but in other 

 relations which he has investigated, there does not appear to be any 

 analogous regularity. 



As regards the values of d, they appear to fluctuate as the series 

 is ascended in the order of complexity of acid radical, but too great 

 dependence must not be placed on the values given. A very small 

 change in Jc would make a considerable difference in the value to be 

 assigned to d, 



An attempt has been made to ascertain whether molecular volumes 

 admit of more regular comparison at temperatures at which mole- 

 cular surface-energies are equal. This appears, however, not to be 

 the case. Thus, at the value 390 ergs, the group of four isomerides 

 gives the following numbers : 



Ester. Molecular volume. 



Methyl butyrate 126'17 



Methyl isobutyrate 125'93 



Ethyl propionate 126'80 



Propyl acetate 127'16 



The agreement is no better than at their boiling points under 

 normal pressure. 



An attempt has also been made to find whether the boiling points 

 at corresponding pressures bear a constant ratio to the temperatures 

 of equal molecular surface-energy. Taking that ratio of pressure to 

 critical pressure given in the fifth line of the table on p. 1245 of the 



iper in the ' Transactions,' and dividing the corresponding tem- 

 perature for each liquid by the temperature of equal molecular 

 surface-energy, the following ratios are obtained : 



