Page:Outlines of Physical Chemistry - 1899.djvu/86

 66 OUTLINES OF PHYSICAL CHEMISTRY

group OH (extra-radical oxygen) has the atomic volume 7'8, whilst oxygen which is doubly linked with carbon (intra-radical oxygen) has a greater atomic volume value, namely, 12*2. For sulphur and nitrogen there are similar variations.

Amongst the most interesting deductions from these observations are the following : (1.) For unsaturated com- pounds the calculated molecular volume is only equal to the experimental after the addition of 4*4 units for each double bond which occurs in the molecule. (2.) The hexahydro- derivatives of benzene and its homologues have a smaller molecular volume than the isomeric defines of the fatty series. The formation *of the aromatic ring is therefore accompanied by a contraction of the molecule.

The large number of experimental facts which have been ascertained by Kopp, Lossen, B. Schiff, Schroeder, and others, have shown, however, that the system of calculation, the basis of which has just been given, is not capable of general application. The atomic volume of all the elements, hydrogen included, is more or less variable. The atomic volume of oxygen may even become nega- tive :

Molecular volume

In an attempt to account for these irregularities, I. Traube finds the causes of failure of Kopp's system in th following considerations :

(1.) A large number of substances form in the liquid state polymerised molecules which are n times greater than the gaseous molecules. The formation of these molecular aggregates being very probably accompanied by a contrac- tion, it would seem evident a priori that no regularity found with such substances would hold good with non- associated substances ; and vice versa*

�� �