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<M mesomorphic conditio_Ns I

reciprocal of mobility, was at one time mi called piasticity, a mistake which has unforhi ^nlV been repeated as recently as in 1937 by q ^t(% and others, who use the term in this sen_s?^<?ⁿ chocolate. Atterberg shows the importance ot t? ¹ moisture rangę over which clays are piastic condition which he calls the " plasticity number^,,a and seyeral workers, chiefly Russian, mistranslat_ei this term as plasticity. The two ideas are different, and are only partly related.

Plastic materials are generally, though n₀i

always, two-phase systems. Plasticfcfroperties often occur in systems in which long,, rod-like molecules orientate themselves in ranks andsrows, the so-called mesomorphic condition. pometimes the lines of molecules fit neatly end to end, the smectic condition ; and sometimes, although orien-tated, their ends do not meet in any regular way, the nematic condition. This is shown diagram-matically in Fig. 14.

Von Buzagh says of this that "a nematic substance is easily mobile (liąuid) in the direction of the longitudinal axis of the primary units and

• _Q11v stable (solid) in other directions. A _mechąnicauy _ce j_{s eas}iiy deformed in a direction smectic s __lanes of the laminse, i.e., the lamina; ^pah« disolaced in this direction. For instance, a ^ctt” _m oleate film consists, as was shown by forin of practically drcular lamellse, in the interior of which Brownian movement can be observed on oarticles of dust, etc. The lamellse are, therefore,

Uquid in the piane of the film ; but they offer considerable resistance to delormation at right angles to the piane of the film.” This idea will occur again when the structure of rubber is discussed. Anisotropic rigidity is quite a common phenomenon.

It is interesting to consider the relationship between plasticity and the fLow curve. Hall, and apparently Wilson also, attempted to measure plasticity as the stiffness, for materials adjusted to constant yield-value, or, alternatively, as the yield-value for constant stiffness. Bleininger echoes somewhat similar ideas, but less definitely. Such a scheme is ąrdte unsatisf actory dimensionally and also does not work in practice. It is not fair to compare days at a constant moisture content, sińce their water absorption capacities are different and the resułts obtained give only a measnre of absorption capacity. Scott Blair has shown, quite empirically, that if the moistures of clays are adjusted so that the yield-vałue of the water envelope in the pług region is constant, then the shearing strengths give a measure of plasticity. Unfortunately, the ratio of shearing strength to adhesion is not qnite independent of the adhesion chosen, unless the clay is wetted in vacuo. In the case where it is ai independent constant, it will also be convenienth dimensionless. Plasticities determined in this wa are called “ flow-plasticities.” The mechanism i

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