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112 STICKINESS

by making the silkworm spin its thre tension. This was done by means of ^ ^Un^er which wound up the tliread while pulling it ^m°^tor, the silkworm. Mark has shown that in geneJT fibrous materials, the longer the molecular _Ch ^°^r the stronger the fibrę. Houwink points out «?^! in the case of some metals such as cadmium Iow temperatures, work-hardening is so great th i a point is reached at which no further deformation occurs at all. Davis finds that this is true also of cheese. It is interesting that fibrous materials are often good adhesives; but the connection is not really understood.

Stickiness seems to be another independent property. Katz points out its great importance in the baking industry, and Zigler has investigated the psychological perception of stickiness. Various workers have measured the stickiness of soils, notably Katchinskii, who makes the measurements with a balance. To one arm of this balance is attached a weight, resting on the surface of the soil, and to the other a pan to which enough load is added to drag the weight off the surface of the soil. Bouyoucos uses a spring balance, which Halton and Scott Blair have adapted for use with flour doughs. Stickiness should be measured by a ąuick dragging away of the adhering body, otherwise yiscous flow occurs, and the cross-section of the area of contact is altered. The Katchinskii balance suffers from this defect: at a certain point on the moisture scalę, the cohesion within the materiał becomes less than the adhesion to the weight, and the materiał pulls out, necks and breaks. The load on the pan is thus no longer a measure of stickiness.

Stickiness measured by such means depends on the time during which the materials have been in

tact tho pressure betwecn thom, and the ratc of application of the load separating them. The laws relating these variables havc not yet been fully worked out. Stickiness also depends on the naturę of the materiał of which the weight is madę, the different metals having different adhesive capacities. This is important in the designing of ploughs, where the object is to shear the soil without causing it to stick unduly to the mouldboard. Herschel and Pisapia have measured the stickiness of Port land cement with an apparatus of the Bouyoucos type.

Another property of materiał which also is independent is oiliness. Herschel has evolved a techniąue for measuring this. Herschel defines oiliness as | the joint property of metals and lubricants which causes a difference in the coefficient of friction when two lubricants or two bearing metals of equal smoothness are used under identical conditions, the conditions being such that the change in viscosity with temperaturę has no effect on the friction.” ¹ Herschel claims that there are several similar but distinct properties, e.g., the property of preventing seizure. Seizure is really welding, and just as dirt prevents welding, • so certain atoms, such as sulphur and chlorine atoms, prevent seizure. There are such problems as that of the prevention of wear, and those of cutting-oil properties. Hersey gives a description of an apparatus for measuring oiliness.

Oiliness must be related to viscosity. In the case of very thin films, a condition about which very little is known, Terzaghi, Scott Blair, Hardy, Grout and Popp, and a number of other workers claim that when such films are composed of such liąuids as water, they do not obey Poiseuille's law. Weber

¹ American Instrument Co. Bułleiin, 136.