APPLICATIONS OF THE VISCOMETJRIC METHOD 287
of plastic flow at every stage. Clays must have a friction high
enough so that the ware will not lose its shape while in the moist
condition and at the same time it must have a mobility which is
high enough so that the clay may be readily worked and it must
not shrink badly on drying. On heating, the more fusible parti-
cles must soften sufficiently to weld the particles together, but
again the friction must be sufficient so that there will be no serious
loss of shape. When the glaze is added, it must fill the pores
quickly and yet not "run." So many problems in plastic flow
seem to call for precise control of conditions in order to avoid
large losses.
It is found that considerable amounts of non-plastic clay, fine
sand, or ground porcelain (grog) may be added to a very plastic
clay without greatly lowering its plasticity. Until more data
is accumulated, this may remain something of a mystery, but
these additions are valuable and probably serve somewhat the
function of the "reinforcing" in concrete or of the colloid in
"solidified alcohol."
Geo-physics.—Basic lavas are notably fluid as compared with
acidic lavas which are more viscous. This has important bear-
ings upon the character of volcanic eruptions in different parts
of the world and presumably therefore upon the past history of
the earth. For example, the Hawaiian volcanoes with a highly
basic lava tend to remain open, flow quietly, build a low-angle
cone, the lava spreading out over a large amount of territory. On
the other hand, the Mexican volcanoes with acidic lava are apt
to harden over during quiescence and then erupt violently. A
low-angle cone is impossible. In accordance with the relationship
between the fluidity of the melt and the rate of crystallization,
we should expect to find the basic lavas more coarsely crystalline
than those of a more acidic nature. The length of time required
for an obsidian to take on a cryptocrystalline, microcrystal-
line or even macrocrystalline character will of course also de-
pend upon the temperature and to a lesser extent upon the
pressure as well as the chemical composition, for all of these
factors influence the fluidity. Silicate melts have been studied by
Doelter (1906).
Segregations, as in the separation of iron from slag, is depend-
ent to a certain extent upon the fluidity of the slag and of the