336 



Mr. Gervaise Le Bas 



The conclusion is that, under the conditions just specified, 

 the differences for CH 2 are not quite constant when the 

 above four normal hydrocarbons are considered. 



If, therefore, the calculation of atomic volumes be attempted 

 from a consideration of the differences, no reliable results can 

 be obtained. This, however, has been the method followed 

 hitherto in the discussion of physical properties in their 

 relation to chemical constitution. It must not for this 

 reason be concluded that the additive rule may not be very 

 closely followed. The calculation of additive relations by 

 differences virtually assumes that as we pass down a series, 

 the addition of the homologous increment CH 2 , e.g., leaves 

 the volume of the residue the same as before, provided 

 that coincidence of physical state is maintained. Although 

 very nearly true at the boiling-point, this is not quite the 

 case, and any change from compound to compound affects 

 every atom in the molecule. The method of calculating 

 from differences throws on to the increment the onus of 

 such changes ; so that very serious disturbances are some- 

 times brought to bear on the volumes of the increments, 

 especially if these are small. Moreover, atomic volumes 

 obtained in this way have never any other signification than 

 that which may be given to numbers which are derived 

 from the average of as many numerical coefficients as 

 possible. The method adopted in this paper does not labour 

 under this disadvantage, for, assuming that the 4 : 1 rule 

 applies very closely between the atoms of any individual 

 hydrocarbon under all physical conditions, the ratios V/W not 

 only show the validity of the above rule for the different members 

 of a series under corresponding conditions, but they also shoiv 

 the extent of constitutive changes from compound to compound. 

 This is demonstrated in a striking way by the following table. 

 A comparison is made of the above hydrocarbons at the cor- 

 responding pressure, '011795 (see Table XIV,). 



Table X. — Volumes of CH 2 at B.P. and at equal reduced 



pressure. 



No. 



Hydrocarbon. 



M.V. 

 at B.P. 



A. 



M.Y. for 

 P/P K =01179 



A. 



6 V/W. 

 21-77 



1 

 1. 



n Pentane, 5 H ]o 



117-80 





116-13 











22-13 





21-06 





2. 



n Hexane, C G H U 



139-93 



22 63 



137-19 



21-49 



21-66 



3. 



n Octane, C 7 H ]G 



16256 



23 70 



158-68 



2183 



21-64 



4. 



n Octane, C 8 H^ 



186-26 





180-51 





21-66 



