By Dr. S. RIDEAL and W.E. YOULE.

Subjoined is a table giving the absolute viscosity of various gums. A comparison of the uncorrected viscosities with the corrected shows the great importance of Slotte's correction for dextrins and inferior gum arabics; in other words, for solutions of low viscosity, while it will be observed to have little influence upon the uncorrected η obtained for the Ghatti gums and the best samples of gum arabic.


Sample.η Uncorrected.η Corrected.Z Water = 100.
Gum arabic0.18760.18561,233
Cape gum0.15750.15551,029
Indian gum0.05400.0470311
Eastern gum0.06890.0639417
Gum arabic0.05500.0480317
Gum arabic0.05110.0430285
Ghatti, 5 per cent0.09030.0828688
Ghatti, 5 per cent0.13910.13501,089
Ghatti, 5 per cent0.17950.17601,420
Ghatti, 5 per cent0.15270.14851,198
Ghatti, 5 per cent0.11390.1083873
Ghatti, 5 per cent0.14190.13691,104
Gum substitute0.03180.0224180
Gum substitute0.03180.0224180

In the column for η corrected the differences due to the use of different instruments are of course eliminated. The absolute viscosity of water at 15° C. determined in four different instruments is shown below. Poiseuille's value for water being 0.0122.

η corrtd. of water.0.01090.011850.01240.0120
K value.0.0000008980.0000008630.0000009320.00000052
K value.0.2350.21750.2260.0204

The above values for various gums and dextrins were obtained at a constant temperature of 15° C. and are compared with water at that temperature. It is of the utmost importance that the temperature of the water surrounding the bulbs should be adjusted for each series of experiments to the temperature at which the absolute viscosity of the water was determined. As far as we have ascertained, in gum solutions there is a steady diminution in viscosity with increase of temperature until a certain temperature is reached, beyond which increase of heat does not markedly 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° C. and 90° C., no gums showing any marked decrease in viscosity between 80° C. and 90° 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° C., was then taken and repeated for every fall of 4° C. till the water reached the temperature of the air.

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. Z = 228

Gum. 30° C. Z = 339

Gum. 20° C. Z = 412

from which the constants--

A = 592.99 B = -10.2153 C = 0.0583

can be obtained, and thus the value of Z 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.