The values so obtained gradually diminished with the increase of temperature. From the η values obtained the Z values were calculated, using water at 15° 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° C and 100° C., while gum sample 4 has a curve bending between 60° C. and 70° C. Experimentally this increase of viscosity of the latter gum above 60° 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 + Ct²
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° C. Z50° = 228
Gum. 30° C. Z30° = 339
Gum. 20° C. Z20° = 412
from which the constants--
A = 592.99 B = -10.2153 C = 0.0583
can be obtained, and thus the value of Zt° for any required temperature. The numbers calculated for gums all point to a diminution in viscosity up to a certain point, and then a gradual increase. A comparison of some of the figures actually obtained in some of these experiments, compared with the calculated figures for the same temperature, shows their general agreement.
Curves showing viscosity change with temperature for three typical gums. A--Arabic VII. B--Senegal VIII. C--Ghatti 15.
EFFECT OF TEMPERATURE UPON VISCOSITY--GUM VII.