1877.] P?*operties of Homologues and Isomers. 247 



The two curves are in this case almost identical. A satisfactory 

 equation has not yet been obtained owing to the absence of any measure- 

 ments at low temperatures. 



In the following Table the densities are compared at those tempera- 

 tures at which the vapour-tensions are equal : — 



Vapour- 

 tension in 

 millims. 



Boiling-point 



of normal 

 propyl iodide. 



Boiling-point 



of iso- 

 propyl iodide. 



Density of 

 normal 

 propyl iodide. 



Density 

 of iso- 

 propyl iodide. 



Difference 

 between 

 densities. 



200 



62-37 



50-50 



1-66704 



1-64590 



0-02114 



300 



73-51 



61-33 



1-64493 



1-62359 



0-02134 



400 



81-95 



69-70 



1-62808 



1-60646 



0-02162 



500 



88-84 



76-44 



1-61446 



1-59246 



0-02200 



600 



94-70 



82-11 



1-60250 



1-58068 



0-02182 



700 



99-83 



87-13 



1-59221 



1-57035 



0-02186 



760 



102-63 



89-86 



1-58670 



1-56497 



0-02196 



The above numbers show that the relation between vapour -tension and 

 expansion, which had been rendered probable by Kopp's researches, does 

 not exist, that the molecular volumes of the two isomers at correspond- 

 ing temperatures are unequal, that of the normal iodide being greater 

 than that of the other by an almost constant quantity. This fact is in 

 accordance with the modern dynamical h) 7 potheses on the constitution of 

 matter. Isopropyl iodide is more unstable than the normal iodide ; its 

 instability may be due to the greater velocity of its molecules ; from this 

 greater velocity would follow a greater mean distance between the 

 molecules. 



I may here mention that I have made a very complete series of ex- 

 periments on the vapour-tension of normal propyl alcohol. The curve 

 representing these observations intersects that which expresses the 

 tensions of normal propyl iodide, so that whereas at 760 millims. the 

 iodide boils at 102°*5 and the alcohol at 97°-3, at 370 millims. they boil 

 at the same temperature, viz. 79°-5, and at 120 millims. the boiling- 

 point of the iodide is only 49°- 5, whilst that of the alcohol is 56°. 



This fact, which probably arises from the much greater latent heat of 

 propyl alcohol, obviously renders useless all attempts to derive the 

 boiling-points from the constitution of chemical compounds, so long as 

 the boiling-points at the ordinary pressure of the atmosphere alone are 

 taken into account. 



