BETWEEN THE VISCOSITY OP LIQUIDS AND THEIR CHEMFCAL NATURE. 589 
nitudes were simply related amongst themselves in the case of non-associated liquids, 
deviations from such regularities would be exhibited by complex liquids, and tliese 
deviations would give some indication of the existence and extent of the molecular 
grouping. 
A. Comparisons of Viscosity Magnitudes at the Boiling-Point. 
Following the suggestion of Schroder and Kopp, the temperature of the ordinary 
boiling-point has been largely used in the comparison of the physical constants of 
liquids ; at this temperature the vapour pressures of the substances are, of course, the 
same, and this equality seemed to justify the use of the boiling-point as a comparable 
temperature. 
Guldberg has pointed out that the reason why Kopp obtained relationships 
between the densities of liquids and their chemical nature at the boiling-point, 
probably lies in the fact that the boiling-point is approximately a so-called corre¬ 
sponding temperature. 
If the absolute boiling-point be divided by the absolute critical temperature, a 
quotient having the average value of 2/3 is obtained, so that the ordinary boiling- 
point is approximately the corresponding temperature of ‘6 6. According to the 
theoretical views of van der Waals, the thermal properties of liquids should be 
compared at such corresponding temperatures, and the accidental agreement between 
the hoiling-point and the corresponding temperature of ’66 is taken by Guldberg 
as being the reason for the success of the boiling-point as a temperature of comparison. 
It may be contended, however, that this argument is not altogether valid. Seeing 
that, in general, the higher the boiling-point the higher is the critical temperature and 
the larger is the difference between the two, the ratio of the absolute boiling-point to 
the absolute critical temperature cannot vary very much for different substances. 
It may be written— 
B.P. -f 273 ^ _ C.T. - B.P. 
B.P. -F 273 + C.T. - B.P. ’ C.T. + 273 ’ 
where B.P. is the ordinary boiling-point and C.T. the ordinary critical temperature 
expressed on the centigrade scale. 
Here the ratio is seen to be unity diminished by a fraction in which the numerator 
increases along with the denominator, which latter contains a large constant term, 
viz., 273. 
According to another deduction from van der Waals’ theory substances may be 
compared when under corresponding pressures, or pressures which are the same frac¬ 
tion of their critical pressures. Comparisons at corresponding temperatures should 
lead to like results. If the boiling-point were a truly corresponding temperature, 
then the vapour pressure at the boiling-point should for all liquids be the same 
fraction of the critical pressure. 
