110 L. V. H E I L B R U N N 



If the moving plane were to travel at two or three times unit 

 velocity, the force exerted on the stationary plane ought to increase 

 two or three times, and this would be true if no elastic forces were in- 

 volved. But, if there is an elastic material between the two planes, 

 as the moving plane traveled at greater and greater speed, part of the 

 extra force it exerted would be expended in overcoming the elastic 

 forces of the material between the two planes, and the force exerted on 

 the stationary plane would not increase proportionately. In such a 

 system, as the moving plane traveled more rapidly and developed a 

 greater shearing force, the measured viscosity would be less. Such a 

 condition is met with in plastic solids such as clay or wax and is also 

 found in many colloidal solutions. For the case of the plastic solid, 

 Bingham (4) proposed the term plasticity, and this term has had wide 

 usage. However, as Barr (3) points out, Bingham does not speak of 

 plastic liquids, and he reserves the term for solids not continuously 

 deformable by a small shearing force It is now customary to use the 

 terms "apparent viscosity" or "anomalous viscosity" for those cases 

 in which the viscosity varies with the shearing force. 



For further discussion of the viscosity concept, see other published 

 works (5-8, 33). 



C. VISCOSITY OF BLOOD 



Ordinary methods of viscometry can readily be used in the study 

 of the viscosity of blood. A standard type of instrument is the Bing- 

 ham viscometer. The technique of measurement with this viscom- 

 eter has been described by Swindells (9) 



Clinically, it is advantageous to have a viscometer that can meas- 

 ure very small samples of blood. The Hess viscometer can measure 

 the viscosity of a single drop of blood. This apparatus can be ob- 

 tained from most of the well known apparatus companies. It con- 

 sists essentially of two capillaries of equal bore and equal length 

 connected by a T tube with a suction bulb. Simultaneously, blood is 

 sucked through one capillary and water through the other. The 

 relative viscosity of the blood as compared to the water is determined 

 from the volume of blood that has flowed through one capillary in 

 comparison with the volume of water that has flowed through the 

 other capillary, the viscosity being inversely proportional to the vol- 

 ume of flow for a given time. 



Figure 1 shows the essential tubes of the apparatus. The two 



