20 



C. L. SADRON 



0.2 



0.1 



centipoises 



Fig. 8. Rigidity test by stream double refractions: tg a is the slope, at zero shear 

 stress, of the isocline curve at different values of solvent viscosity 770 . Above: behavior 

 of a (lower curve) rigid particle (TMV) and of a (upper curve) flexible coil (poly- 

 styrene in cyclohexane, chain molecule). Below: behavior of DNA particles (in water 

 plus increasing percentage of glycerol). At low concentration of glycerol these par- 

 ticles behave as rigid bodies. 



tations of the many, very confused data on the weight and shape of the 

 dispersed DNA particles been subjected to clear criticism. 



It seems that we are now in a much better situation for obtaining signifi- 

 cant results, and it is beyond any doubt that in the near future we shall be 

 able to benefit from data that will deserve careful consideration. At the 

 present time, however, we are still constrained to adopt a rather cautious 

 position. The main objections which have been put forward are related to 

 two very different points. 



The first is connected with the validity of the light-scattering measure- 

 ments, and the second with the reproducibility of the DNA preparations. 



Though the latter question seems logically to be the first to be answered, 



