BETWEEN THE VISCOSITY OF LIQUIDS AND THEIR CHEMICAL NATURE. 023 
other series of slopes, provided that the method of choosing is the same throughout. 
This last point is treated in detail at a later stage. 
The comparisons about to be given are made at temperatures at which drjidt is the 
same for the different liquids. Or graphically, the temperatures may be defined as 
those corresponding with points on the viscosity curves at which tangents are 
equally inclined to the axes of coordinates. The temperatures are therefore those at 
which temperature is exercising the same effect on viscosity, and for shortness may 
he termed temperatures of equal slope. 
The temperatures were obtained by means of Slotte’s formula. 
On differentiating the formula in the shape 
p = 
(« + ty 
we obtain 
drj 
dt 
TIC 
{a + + i ■ 
On choosing a particular value of — dpjdt, say S, values of t corresponding with 
this value of the slope were deduced from the equation 
t = (nc/S) 
i/(« + ]) 
1 / , \ loo’ nc — loo; S 
or log(a + 0= 
Slopes at which Comparisons loere made. 
The first point to decide was what particular value of the slope should be 
employed. It was apparent from the shape of the curves that all the liquids could 
not be compared at any one value of the slope, because the effect of temperature on 
viscosity varied so much from substance to substance. In some cases, the whole of 
the alcohols for example, the slojae at the boiling-point was considerably greater than 
that at 0° in the case of some of the less viscous liquids. 
The question then resolved itself into choosing, in the first instance, a slope at 
which the viscosity of as many liquids as possible might be compared. From a study 
of the curves the particular value fixed upon was S = '0000323, which happens to 
be the value possessed by methyl iodide at its boiling-point. 
Comparisons were then made at the slope '0000987, which was chosen in order 
that the liquids which could not be included at the smaller slope might be compared 
with as large a number as possible of the other liquids. 
The values of the temperature at these slopes, and of the specific molecular areas 
and specific molecular volumes for the various licpiids are given in the appendix, Tlie 
following tables contain the various viscosity magnitudes at this system of comparable 
temperatures for as many liquids as could be compared at each value of the slope. 
In some cases the curves have had to be extrapolated for a short distance beyond 
the ordinary range from 0° to the boiling-point. 
