ly influence the viscosity, and it is possible that above
this "critical point," as we may term it, the gum solutions once more
begin to increase in viscosity. The temperature at which the viscosity
becomes stationary varies somewhat with different gums, but broadly
speaking it lies between 60 deg. C. and 90 deg. C., no gums showing any marked
decrease in viscosity between 80 deg. C. and 90 deg. C.

The experiments we have made in this direction were conducted as
follows. The 300 c.c. bottle containing the gum was placed in a
capacious beaker full of hot water, and the viscosity instrument was
also surrounded with water at the same temperature. Thermometers were
suspended both in the beaker and the outer jar. The viscosity at the
highest temperature obtained, about 90 deg. C., was then taken and repeated
for every fall of 4 deg. C. till the water reached the temperature of the
air.

The values so obtained gradually diminished with the increase of
temperature. From the [eta] values obtained the Z values were
calculated, using water at 15 deg. C. as a standard. From the Z values thus
obtained taken as the ordinate, and the temperature of each experiment
as the abscissa, curves were plotted out embodying the results, examples
of which are given below. The curves yielded by three gums 2, 7, and 8
changed between 90 deg. C and 100 deg. C., while gum sample 4 has a curve
bending between 60 deg. C. and 70 deg. C. Experimentally this increase of
viscosity of the latter gum above 60 deg. C. was confirmed, but the critical
point of the other solutions tried approaches too nearly to the boiling
point of water for experiments to be conducted with accuracy, as the
temperature of the bulbs diminishes sensibly while the experiment is
being made.

If viscosity values have been determined it is possible to calculate the
remaining or intermediate values for Z at any particular temperature
from the general equation--

Zt = A + Bt + Ct squared

As an example of the mode of calculation we may quote the following. A
gum gave the following values for Z at the temperature stated:

Gum. 50 deg. C. Z_{50 deg.} = 228

Gum. 30 deg. C. Z_{30 deg.} = 339

Gum. 20 deg. C. Z_{20 deg.} = 412

from which the constants--

A = 592.99 B = -10.2153 C = 0.0583

can be obtained, and thus the value of Z_{t deg.} for any required
temperature. The numbers calculated for gums all point to a diminution
in viscosity u