on the Physical Properties of Compounds. 113 



and methyl formate have both exactly the same composition, 

 viz. C 2 H 4 2 , and yet have very different properties, — the 

 former being a well-characterized acid, which on cooling soli- 

 difies in the form of large transparent plates, which melt at 

 + 17° and boil at 118° 0., it possesses a pungent sour smell, 

 and is highly corrosive; whilst the latter is a colourless liquid 

 which has not yet been obtained in the solid state, and boils as 

 low as 33°'4, or about 85° below the isomeric acetic acid, 

 from which it also differs notably in emitting a pleasant 

 ethereal smell. Since both compounds have exactly the same 

 composition, this remarkable difference in properties can only 

 be caused by the atoms being arranged differently in the two 

 bodies; and that the atoms are differently arranged, we know 

 from the various chemical decompositions which the two com- 

 pounds undergo. This difference in atomic arrangement is 

 represented by the following graphical formulas : — 



H H 



I I 



H-C— H H— C— H 



I I 



0=C— 0— H 



I 

 0=C— H 



Acetic acid. Methyl formate. 



In the former the two carbon atoms are directly united, whilst 

 in the latter they are combined indirectly by means of oxygen. 



I. Influence of Atomic Arkangement on the Melting- 

 point. 



It seems strange that scarcely any attempt has been made 

 to trace the connexion between the composition and melting- 

 points of carbon compounds, notwithstanding that the melting- 

 points of a very large number of such compounds have been 

 determined. The most noteworthy of these attempts is that 

 of Baeyer, in regard to the melting-points of the fatty acids, 

 and that of Korner, who states that, amongst aromatic com- 

 pounds, what are called para-derivatives usually melt higher 

 than the corresponding ortho- and meta-derivatives. 



Attention has been directed almost exclusively to the boil- 

 ing-points. Now the boiling-points are by no means so suit- 

 able for tracing the influence of atomic arrangement as the 

 melting-points, since they are far from being such a fixed 

 property as the latter; for, as is well known, the boiling-point 

 is raised or lowered considerably by even small alterations in 

 the pressure, whilst under ordinary circumstances the melting- 



