  A liquid obeys isochronal superposition if its dynamics is invariant along
the isochrones in the thermodynamic phase diagram (the curves of constant
relaxation time). This paper introduces two quantitative measures of isochronal
superposition. The measures are used to test the following six liquids for
isochronal superposition: 1,2,6 hexanetriol, glycerol, polyphenyl ether,
diethyl phthalate, tetramethyl tetraphenyl trisiloxane, and dibutyl phthalate.
The latter four van der Waals liquids obey isochronal superposition to a higher
degree than the two hydrogen-bonded liquids. This is a predic- tion of the
isomorph theory, and it confirms findings by other groups.
