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130 RHE0L0GICAL METHODs

one can measure the deformations, eithe or, better still, by extending a sample on a ^rollUi bath; or the gravity method may be us_P?^erci4 deformations should be divided into the tj % or plastic part and the recoverable or elas?^⁶^ remembering that for materials which. sho\v ° after-effect, this distinction is arbitrary. ||Sl| hardening, elastic hysteresis and after-effect °^rk not be forgotten. If materials are diilatant, th little that can be done to obtain adeąuate riieoloe? S data.    ^§lc^

(iv)    For materials which are , still morę nearlv solid, one can load with a weight for a fixed time allow them to recover, and calculate the viscosity and the shear modulns, remembering that the elastic after-effect may be very slow and that one must allow time for this. By loading with two weights, one can obtain data from, which th^ ^Juctural viscosity can be worked out; and by making com-parison for the same load at different strains the work-hardening can be measured. It b snmetimes effective to load a cylinder, hung from Wgid support.

(v)    In generał it is advisable to try,, as far as is' possible, to get measurements of ■ recognised pro-perties in absolute units. It may sometimes appear that it gives less trouble to measure empirical mixtures of properties, and fhose directors|Of industrial firms who are unfamiliar with scientific research will sometimes think that Stach methods save valuable time; but in practice it is very ,likely to prove unsatisfactory, and in the long run to waste morę time than really fundamental work.

(vi)    Sometimes quite novel methods have to be devised. For example, in the poultry industry a method was required of measuring the. eonsistency

I _eggs without breaking them. The experimenter ¹ took a torsion wire, fixed a rod across the middle of +he wire to make a pendulum, attached the egg to the rod, and let it swing. If the contents of the egg were truły fluid, the damping would depend on the mass of the egg and on the viscosity. In dealing with eggs this gives a good over-all measure of ćonsistency, though Wilcke, who devised the method, described the property measured as yiscosity. This is an excellent instance of an ingenious solution to a difi&cult industrial problem.

The author makes no apology for once again emphasising the diversity of materials for which rheological methods are being used, and the essential unity of the problems involved. In the United States of America the Society of Rheology is making considerable progress in co-ordinating the activities of those engaged in these different industries, though even now, much remains to be done; but in Great Britain and in most European countries little or no organisation exists for this purpose. Work of generał rheological importance is published in ^ obscure trade and technical joumals which are not readily available to those engaged in other industries: and the great majority of young chemists, on whom the brunt of technical rheological work will fali, are drafted into industry from collegęs and schools with little or no training in rheological methods.

A colleague from abroad. recently commented on the family spirit which exists among rheologists, so many of whom are known to each other personally. This is an encouraging fact in these troubled days; but it must be regretfully admitted that there are thousands of workers who should belong to the

¹ Wilcke: Iowa Stałe College Research Bulletin. No. 194. I93⁶-

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