78 METHODS FOR DETERMINING MOLECULAR SIZE AND SHAPE 



Recent developments of the electrophoretic technique include placing 

 the solution on long strips of special filter papers in a moist atmosphere, 

 and applying the electric force across the ends of the paper strip. The 

 separation of the constituents is then achieved simply by using a scissors, 

 and the constituents may then be eluted from the paper in suitable 

 buffers. 



(b) Static methods 



If the particles are stationary, then the physical agent for extracting 

 information has to move into and out of (or at least away from) these 

 particles. Thus, we are led to examine the irradiation of particles by 

 visible light, ultraviolet and x-radiation, and various charged elementary 

 particles. 



Visible light scattering. If a particle is very tiny, light waves scattered 

 off its front and rear will be so nearly in the same phase that they 

 scarcely interfere with each other in the sense of waves reinforcing or 

 cancelling each other. If, as indicated in Fig. 36, the particle is compara- 

 ble in size to the wavelength of light being used, then the wave scattered 



Eye or 

 photographic film 



Fig. 36. A light beam whose wavelength is comparable to the dimensions of 

 the scattering particle is reflected in various directions by the particle. The 

 bottom wave travels about a half wavelength more than the middle wave be- 

 fore being reflected, so that it is possible to find a direction, as indicated, in 

 which the eye or a light-sensitive film will record that the reflected beams arrive 

 out of phase, so that there is a cancellation of beams. Other directions could be 

 found in which the beams arrive so that thev reinforce each other. 



