Page:Outlines of Physical Chemistry - 1899.djvu/90

 70 OUTLINES OF PHYSICAL CHEMISTKY

for each polymerised particle (consisting of n times m), and we should then have

~ = 2 atomic volumes +

a n

The Association Factor. — When the molecular weight of a substance is known, the experimental determination of the

co-volume ( — - — 2 atomic volumes) may be used as a

means of ascertaining the degree of association. If at 15° the co- volume is, then the liquid is com-posed partially or wholly of polymolecular particles, and the association factor is x. The following are some values of x determined in this way (which do not in every case agree with the values found by Bamsay's method) : for glycol, 2; for glycerine, 1.75; for formic acid, 1.67 ; for acetic acid, 1.40; for methyl alcohol, 1.66; for ethyl alcohol, 1.51. Acetone, acetonitrile, and propionitrile are also associated. Hydrocarbons and ethers are as a rule very little associated. Of all substances, water has the highest association factor (higher than 3).

Bemark. — In Traube's later publications he dis- tinguishes :

(a) the material nucleus of the atom, and (b) the space in which this nucleus performs its oscillatory motions.

The constants on page 68 are composed of the sum of the volumes (a) and (b), but may continue to be called simply the atomic volumes.

The atomic volumes of several elements appear not to be very constant. Thus the value found for nitrogen varies between tolerably wide limits : from 1.5 (in certain primary amines) to 18 (in tri-isobutylamine). Similar variations have been detected for chlorine, bromine, &c., and this want of constancy is certainly a great drawback to the practical value of the system.