The Normal Viscosity of Pure Liquids

Apart from surface forces and compressibility, observations and measurements show that the differential equations of hydrodynamics in their 

See particularly the book Die hochmolekularen organischen VerbindungeUt Berlin 1932, p. 56, 489 etc. Also more recently Kolloid-Z.j 82, 129 (1938). 

Newton was the first to realize that the force corresponding to this performance of work against internal friction behaves entirely differently from that operating in external friction he assumed that the shearing force of internal friction r is proportional to the surface /, and to the relative velocity u of the liquid planes in relative motion. By conversion to the limiting value for infinitely thin planes, he was able to write 

The proportionality constant rj in this Newtonian Law bears the name viscosity its dimensions are 

The unit viscosity in absolute (cgs) units—one poise—is possessed by any liquid in which the force of i dyne is neceijsary to maintain a velocity gradient of 1 cm per sec per cm perpendicular to a surface of one cm. In this case also the power expended is 1 erg per sec. 

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