320 FLUIDITY AND PLASTICITY

factory. A more certain method is to platinize the glass and
then solder in position with pure tin as solder.

To determine the effect upon the flow of changing the length
and radius of the capillary, at least four capillaries may be
required, hence it is convenient to keep all of them mounted
continuously in duplicate bases.

Since the mercury thread gives only the radius of a cylinder
which would have the same volume as the capillary, the true aver-
age radius for flow purposes requires more elaborate estimation if
absolute measurements of flow are to be made and any consider-
able accuracy is desired. In fact, since the flow varies as the
fourth power of the radius and it is measurable to the desired
accuracy only with difficulty, it may be said that this is the most
difficult part of absolute measurement.

Whereas one will seek to obtain a capillary tube which is a
true cylinder, it will usually be slightly elliptical. In this case
the ratio of the major to the minor axes may be obtained by
measurements of the -photomicrograph of the ends, although
several other methods may be used. Prom this ratio and the
average radius obtained by means of the mercury calibration,
the actual values of the major and minor axes 25 and 2C may
be calculated.1 It follows then that

C2"
If the capillary is a f rustrum of a true cone,

4 3fli3fl23

T RS+R&2 + RS

where #3 and R% are the radii of the two ends. If the capillary
is not only conical but elliptical at the same time,

(1 - e2)3

'

r> __ ri
where #3 and JR4 are the average radii of the two ends, e = n _, x?
and 25 and 2C are the mean major and minor axes.
THE MEASUREMENT OF PLASTICITY
Until the pressure is admitted the flow by seepage will ordi-
1 Cf. RUCKER, Phil. Trans. 185A, 438 (1894) and KNIBBS, J. and Proc.
Roy. Soc. New South Wales 29, 77 (1895); 30, 186 (1896).