113 DIRECT PROPERTIES OF MOLECULES 311 



stance is in the liquid state, and consequently in a state 

 that is marked by its very slight compressibility. We may 

 therefore assume that the substances in the liquid state 

 have attained nearly the smallest volumes to which they 

 can be compressed. But the specific volume of a molecule 

 in the liquid state is then exactly the same as that other 

 volume which encloses the sphere of action, and which we 

 have also denominated the molecular volume. The single 

 difference that can still exist is due to the choice of the 

 units in which the numbers are expressed ; but this differ- 

 ence also comes to nothing if we content ourselves with 

 relative values and do not strive after a knowledge of the 

 molecular volumes in absolute measure. 



When molecular volumes were calculated for the liquid 

 state from the molecular weights and the specific gravities, 

 simple relations were found between the calculated values 

 and the chemical composition of the substances. Kopp, 1 

 Schroder, and others were led to propose empirical laws, 

 from which the molecular volume of a liquid compound can 

 be calculated by simple addition of the values of the specific 

 volumes of its components. 



Loschmidt 2 and Lothar Meyer 3 found similar and 

 just as simple relations when they attempted to estimate 

 the molecular volumes of gases. For this purpose they 

 started from a knowledge of the molecular free paths and 

 of the diametral sections as deduced on the kinetic theory 

 from observations on the diffusion and viscosity of gases. 

 In order to estimate the size of the molecular volume from 

 the section of the sphere of action they neglected the dis- 

 tinction between the sphere of action and the molecular 

 volume, and therefore took the sphere of action as actually 

 spherical. 



With this assumption it is very easily possible to com- 

 pare the volumes of the spheres of action or the molecular 

 volumes 



1 Ann. Cliem. Pharm. 1855, xcvi. pp. 1, 153, 303 ; 1856, c. p. 19. 



2 Wien. Sitzungsber. 1865, Hi. Abth. 2, p. 395. 



3 Ann. Cliem. Pharm. 1867, 5. Suppl.-Bd. p. 129. 



