BETWEEN" THE VISCOSITY OF LIQUIDS AND THEIR. CHEMICAL NATURE. 587 
In the past, owing to the fact that the tempei’ature variation lias been little studied, 
it has been customary to employ one and the same temperature for all substances. 
The kind of relationships which are obtained by such a system of comparison have 
already been indicated in connection with the graphical representation of our obser¬ 
vations. We have thought it needless to say anything further regarding this 
method, for, as has been shown, the viscosity curves, even in the same family of 
substances cross one another, so that quantitative relationships obtained at any single 
temperature of comparison can have no pretensions to generality, but must vary 
with the value of the particular temperature selected. 
The first comparable temperature which suggested itself was the boiling-point, and 
the detailed examination of viscosity at the boiling-point is first set out. With such 
data as could be obtained relating to critical temperatures, we then calculated values 
of corresponding temperatures by the method indicated by van der Waals. The 
particular temperature adopted (‘G) was such that it included the greatest number of 
cases. A very brief summary of the results obtained at the corresponding tempe¬ 
rature of '6 is next given. Following this are the results obtained by the use of 
a new system of deducing comparable temj)eratures, the details of which are given 
on pp. 622 and 623. 
Viscosity Magnitudes dealt until at the Diffei'ent Te'inperatures of Comparison, 
At each of the different conditions of comparison the experimental results have 
been expressed according to the same system, in order to show at a glance relation¬ 
ships between the magnitudes of the viscosity constants and the chemical nature of 
the substances. The liquids are arranged so that chemically-related substances are 
grouped together—groups of homologues, chlorides, bromides, and iodides, propyl and 
allyl compounds, groups of isomers, kc. The alcohols, on account of their peculiar 
behaviour at all the conditions of comparison, are kept more or less separate from the 
other liquids. Tables are constructed in this way which give the values of three 
different magnitudes derivable from measurements of the viscosity of the substances. 
(1) Values of the Viscosity Coefficient, (y.) 
The first set of tables contains values of the viscosity coefficient in dynes per sq. 
centim. and exhibit how this phvsical constant varies from lic|uid to licpiid at the 
temperature of comparison. 
(2) Vakies of y X Specif-c Molecular Area. The Molecular Viscosity, (yck.) 
The second set of tables contains values of the coefficients y treated so as to 
quantitatively connect them with the chemical nature of the substances. The absolute 
coefficient y is the force in dynes which has to be exerted per unit area of a liquid 
surface in order to maintain its velocity relative to that of another parallel surface at 
4 F 2 
