78 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



assumed, and is certainly not applicable to the linear molecules of 

 cellulose. In these cases the frictional constant may be identified with 

 that occurring in the equation 



D^RTIF, 



where D is the diffusion coefficient of the same solute. Substituting 

 this value in (3), 



M=RTsID(1-vq). ..(4) 



Experimentally the method involves the observation of the movement 

 of the boundary between solution and pure solvent as this moves down 

 the tube. This is done by optical methods into which space will not 

 allow us to go; they can be obtained from any of the standard works on 

 sedimentation. If the solution contains two types of sedimenting par- 

 ticles, then these will move down the tube at different rates, giving two 

 boundaries which will move apart. There are naturally limits to the 

 number of components which are resolvable, but nevertheless this 

 method does yield valuable data on the composition of the more 

 important fractions in a colloidal solution. 



(c) Viscosity determination. — During the past few years a great deal 

 of attention has been paid to the possibility of determining molecular 

 weights from the viscosity of solutions, particularly in view of the ease 

 with which viscosity determinations can be made. As with the other 

 two methods described, this has involved the development of theoretical 

 approaches into which we cannot go here. Again these can be obtained 

 from the relevant literature. Briefly the viscosity of a liquid is that 

 property which is responsible for the internal resistance offered to the 

 relative motion of different parts of the liquid. In a solution, this 

 resistance will clearly be affected by the features of the solute, including 

 the size of the dissolved particle. In order to define the coefficient of 

 viscosity it should be recalled that for pure liquids Newton assumed the 

 shearing force, t, between two parallel planes of liquid in relative 

 motion to be proportional to the area of contact and to the velocity 

 gradient dvldx between them, giving his well-known relation 



r=rjdvldx . A. ..(5) 



The factor rj is then a constant called the coefficient of viscosity, a 

 property of the liquid and independent of the conditions used for its 

 determination. The determination may be made by any one of a number 

 of methods. 



If, for instance, the hquid is caused to flow along a capillary tube 

 / cm. long and a cm. diameter under a pressure difference of P between 



