^8 



must be structure, and perhaps the most reasonable assumption 

 is a form of network in three dimensions, the interstices being filled 

 either with pure solvent or with solvent containing disperse phase 

 in a lower state of aggregation. There are admittedly difficulties in 

 such a hypothesis, e.g., in regard to the equilibrium between the 

 two concentrations co-existing^^. This view of the viscosity of 

 nitrocellulose solutions draws a distinction between them and the 

 highly viscous solutions of such crystalline substances as cane sugar 

 in water, which may be regarded as owing their viscosity to the 

 presence of large aggregates consisting of molecules of sugar surrounded 

 by attracted water molecules. This suggests another contrast between 

 the two. The attraction of sugar molecule for water molecules is 

 regarded as being related to the high solvent power of water for 

 sugar and also to the high viscosity of the solution. In the case 

 of cellulose esters, as we have already seen, the best solvent is regarded 

 as the one which gives solutions of least viscosity. 



In view of these considerations, a study of the transition from 

 emulsoid to suspensoid solutions would be of great interest. It is 

 well knoAvn that an alcoholic solution of mastic, If poured into water, 

 yields a susjDensoid solution of mastic. In a sinular way a dilute 

 acetone solution of nitrocellulose on dilution mth water yields a 

 suspensoid solution of nitrocellulose. From some incomplete experi- 

 ments it appears that the gradual addition of water to the acetone 

 solution, keeping the concentration of nitrocellulose constant, produces 

 a gradual rise in the viscosity of the solution, both absolutely and 

 relatively to the viscosity of the solvent, until a maximum is reached, 

 after which the viscosity falls untU it almost coincides with that of 

 the solvent. The TjmdaU effect becomes marked close to the position 

 of maximum viscosity. On the network view of the structure of 

 nitrocellulose solutions these changes would mean a stiffening and 

 probably a shrinkage of the network as the percentage of water in 

 the solvent increases, and finally rupture into free particles, possessing 

 a refractive index sufficiently different from that of the medium to 

 show the TjTidaU effect. 



Reference should be made to the analogy developed in text-books 

 of physics between viscous stress in Uquids and shearing stress in 

 a strained elastic soUd, according to which a viscous Uquid is regarded 

 as able to exert a certain amount of shearing stress, but is continually 

 breaking down under the stress. The equation developed is tj = n/A 

 in which 77 is the co-efficient of viscosity, n the co-efficient of rigidity, 

 and 1/A the time of relaxation of the medium, i.e., the time taken 

 for the shear to disappear from the substance when no fresh shear 

 s supplied to it. This is a much more illuminating way of regarding 

 the viscosity of liquids than the analogy with gases, since the viscosity 

 of liquids, unlike that of gases, diminishes with rise in temjjerature.-^ 

 But it is particularly suggestive in the case of solutions of colloids 

 like nitrocellulose and rubber which depend for their value on their 

 mechanical properties'*". In the case of celluloid, in particular, we 

 are dealing with what is at ordinary temperatures an elastic solid, 

 and it would be most instructive to follow up its properties from dilute 

 solution to the solid state, both with and without the addition of 



