440 
KEPOKT OF CHEMICAL LABOKATOKY 
Strength of 
solutions 
employed 
will not tlow' through a capillary tube such as must be employed if the determination 
of viscosity of a 10 per cent, solution is made with anything even approximating 
accuracy. 
In our own experience the determinations of the viscosity of gum in 10 per cent, 
solutions has often furnished results markedly at variance with the results obtained in 
higher concentration. This is apparently especially the case with comparatively fresh gum, 
that is, from one to three months after collection. It is probably connected with the 
presence of the variety which, in higher concentration, shows itself by forming a glairy, oily 
liquid, not a true solution, the determination of the viscosity of which has probably little 
practical value, and the figures of which are certainly not comparable with those yielded 
by normal gum solutions. 
If determinations of viscosity are made in 10 per cent, concentration, a solution of 20 
or 30 per cent, should also be made up and allowed to stand for from 24 to 48 hours in order 
to determine whether there is any alteration in the solubility of the gum at the end of 
that time. 
From enquiries made of gum handlers in Trieste, Bordeaux and America, it was found 
that the results of viscosity determinations made on 20 to 30 per cent, solutions were 
considered much more satisfactory, being more in accordance with the behaviour of the 
gum in actual use. 
Oslwald's 
viscosimeter, 
a simple form 
for rough 
determinations 
Terms in which 
viscosity is best 
recorded 
The cost of the torsion viscosimeter is considerable and its setting up and adjustment 
necessitates a certain amount of skill in the operator. It is desirable, therefore, that some 
simple form of instrument lie devised for general use among gum dealers 
and manufacturers. Any one of the forms of flow apparatus will answer for 
such rough determinations provided it is adjusted to the concentration of 
solution to which it is applied. The best of these, and a form cajjable 
of furnishing very accurate results, is that recommended by Ostwald, and 
shown in Fig. 217. The determination is made by introducing a definite 
fixed quantity of the solution at C, and drawing it up by suction until it 
stands at the mark A. The time reijuired for the liquid to flow' from A 
to the lower mark B is noted. The ajiparatus should stand in a tall beaker b 
containing water at the standard temperature. 
The upper small bulb may conveniently have a capacity of 5 c.c., and 
the tube of fine bore a length of 10 centimetres. 10 c.c. of the liquid to 
be tested shoidd be introduced at C. 
The tube should loe of such a bore that the time of flow should be 
between 50 and 100 seconds. 
Whatever the form of instrument adopted, it is desirable to record 
the viscosity in terms of the number of grammes of pure dry cane sugar 
per 100 c.c. of solution (made at 60^ F.) which is required to produce 
the same viscosity at the temperature selected. All the viscosities 
mentioned in this article were determined at 90° F., which was chosen as 
being about the average temperature of a room in Khartoum. 
As will be seen from the curve of viscosities of cane sugar solutions which is charted 
on page 439, the expression of viscosities of gum solutions in terms of gramme-sitgar 
degrees is much more satisfactory with ten per cent, solutions than with those of 
twenty per cent, or more, as the curve beyond 20° retardation (which is not likely to be 
exceeded by even the strongest gum in ton per cent, solution) is very steep. 
Ostwald s viscosimeter 
