64

FLUIDITY AND PLASTICITY

I ra

its middle point until it attached itself to the capillary all of the
way around, the greatest care being taken not to decrease the
diameter of the capillary or change it in any way.

Ajiother marked improvement was the introduction of the
traps Tl and T2, Fig. 22, for the purpose of easily adjusting the
total volume of liquid within the instrument, which they denoted
; 0 as the'' working volume'' to distinguish it from

the volume of efflux V. By keeping the work-
ing volume constant, the correction for the
hydrostatic pressure within the instrument is
greatly simplified.

Unfortunately Thorpe and Rodger's instru-
ment has not come into general use. This is
probably due to the following disadvantages:
The sealing of the wide tube to the middle of
the capillary is difficult to accomplish; and
according to Knibbs (1895) and Blanchard
(1913) one cannot be sure that the bore of the
capillary has not been altered in spite of the
utmost precaution. It is practicable to get
the dimensions of the capillary and the other
constants of the instrument only after the
sealing has been, completed. To get them then
is a matter of some difficulty if not uncertainty.
The instrument is difficult to clean and dry
on account of the narrow spaces between the
capillary and the wider tube at either side of
the constriction R. At the same time the
instrument is rather fragile. These difficulties
may all be overcome by using ground glass
joints between the capillary and the two
limbs. Over the ground-glass joints rubber tubing may be
stretched and tied and thus any danger of a leak guarded
against. The absence of a leak can be proven very easily at any
time by simply testing the working volume. By having good
ground-glass joints there can be no change in volume due to the
change in the expansion of the rubber under the changing head
and there can be no solvent action except that on the glass itself.
On the other hand, by planing the ends of the capillary off at

PIG. 22.—Viscom-
etefr of Thorpe and
Rodger.