224 PROTOPLASM 



We see how quickly through the colander 

 The wines will flow; on the other hand, 

 The sluggish olive-oil delays : no doubt, 

 Because 'tis wrought of elements more large, 

 Or else more crook'd and intertangled. 



How near right Lucretius may have been is shown by the 

 recent work of Staudinger, which indicates that in a solution 

 containing very long, threadUke molecules, normal movement is 

 impeded, and high viscosity results. On this basis, he calculated 

 the molecular weight of substances (rubber), presuming a very 

 simple and direct relation to exist between viscosity and molecular 

 size. That such a relation may exist is obvious, but high viscosity 

 is not necessarily proof of large and linear molecules. Solutions 

 of small molecules are often extremely viscous. The high 

 viscosity and especially the irregular behavior of colloidal sub- 

 stances may be due to the aggregation of their molecules— in 

 other words, to colloidal structural properties. Bingham inter- 

 prets the high viscosity of substances with low molecular weights 

 as due to "association" caused by "associating groups" such as 

 NH2', OH', and COOH'. Because of association, diethylene 

 glycol, C2H4(CH)2, is very viscous, and urea, CONH2, is a solid, 

 as is also oxalic acid, (COOH) 2, with an extremely high melting 

 point. Propionic acid, CHs-CHa-COOH, is a fairly mobile 

 liquid, but hydracrylic acid, CH20H-CH2COOH, is a thick, 

 syrupy liquid. This leads us to the interesting anomalous 

 behavior of some solutions. 



In 1685, Isaac Newton announced, in his "Principia," the law 

 of fluid flow. He expressed it mathematically thus: v = 4>Fr, 

 which means that the velocity, or rate of flow, v is proportional 

 to the fluidity </> (the reciprocal of the viscosity) and the shearing 

 stress F on either of two planes separated from each other by the 

 distance r. Newton had confidence in his theory and never 

 bothered to test it out experimentally, nor did anyone else for a 

 century and a half. It remained for Jean Poiseuille, French 

 professor of medical physics, to furnish proof of Newton's law. 

 He found that the volume of flow of a liquid v is proportional to 

 the time t, the pressure p, the fourth power of the radius of the 

 capillary r, and a constant k, and inversely proportional to the 

 length of the tube I. This relationship he expressed in the follow- 

 ing formula which bears his name: 



