RICHARDS. — SIGNIFICANCE OP CHANGING ATOMIC VOLUME. 599 



many interesting groups of compounds, that having the common formula 

 C7 IIu Oo is especially complete and comprehensive, the boiling points 

 ranging from 224° with normal heptylic acid to 121° with isopropyl iso- 

 butyrate, while the densities range from 0.938* to 0.87'J. Among the 

 seventeen compounds in this group whose data are to be found in Beil- 

 stein's invaluable handbook, the density of only one (methyl caproate) 

 deviates more than 0.005 from its place in the parallel series, the density 

 steadily diminishing as the boiling point diminishes. This is shown in 

 the following diagram, where centigrade boiling points are recorded as 

 abscissae, and densities at 0° as ordiuates. Clearly the correspondence 

 of density to boiling point is too close to be the result of accident. 



BoiLixG Points and Densities of Substances C7 H^^ Oj. 



94 



93 



92 



91 



90 



89 



88 



170 



180 



190 



200 



210 



220 230 



120 130 140 150 160 



The divergent point ou the ordinate 150° represents methyl caproate, and that 

 less divergent on the ordinate 139°, isoaniyl acetate. 



The other substances, named in order, are isopropyl isobutyrate, isopropyl buty- 

 rate, propyl isobutyrate, ethyl isovalerate, butyl propionate, propyl butyrate, ethyl 

 valerate, isobutyl propionate, amyl acetate, methyl isobutyl acetate, hexyl form- 

 iate, isomylacetic acid, isoheptylic acid, iso-onanthic acid, and heptylic acid. 



This regularity is so obvious that it must have been noticed before; 

 but nevertheless, the plausible hypothetical explanation of it, depending 



* This value is Franchimont's (Ann. Chem. Phys., 165, 242 (1873) corrected to 

 0° with the help of Zander's value for the coefficient of expansion. Ibid., 224. 70 

 (1884). Franchimont's value is chosen because his substance was the purest, 

 having a melting point of —8°. 



