In general the characterization of a virus particle depends upon a number of known and independently acquired parameters, all of which require a great deal of time. As a direct method of measurement, the mass of viruses has been largely ignored in virus characterization studies. Where the molecular weight of viruses has been determined by sedimentation and diffusion, sedimentation equilibrium, light scattering, or the electron microscopy-counting procedure, the relationship between this property and mass has been arrived at by employing Avogadro's number. In the present report it will be demonstrated that the disadvantages found in these usual methods of determining molecular weight, such as experimentation, calculation of virus size, and purification of virus preparation, are not present in the quantitative electron microscopy method for determination of mass and molecular weight.

To a certain degree, the events underlying the action potential are understood. For each ion to which the membrane is permeable, there exists an equilibrium potential, whose physical origin is in the activity gradient for that ion. The membrane potential is then the summation of these individual ionic batteries each weighted according to the membrane permeability to that ion. In nerve fibres at rest the potassium permeability is relatively high and the membrane potential is near to the equilibrium potential for potassium. During excitation there is a transitory increase in the permeability to sodium and a slower transitory increase in the potassium permeability also. The membrane potential, therefore, temporarily moves to a value near to the sodium equilibrium potential and then returns to its resting value. Less well understood are the molecular mechanisms responsible for these selective changes in membrane permeability, i.e. how it is that a channel (pathway bywhich an ion traverses the membrane) changes its availability for ion passage as a response to a change in the membrane potential. This is the gating process.

Theoretical methods of conformational analysis have been applied to the biopolymers with beneficial consequences for the elucidation of structure–function relationships in these molecules. The methodology of theoretical conformational analysis and the fruits of its application to the biopolymers, particularly the polypeptides, have been the subject of numerous reviews (Ramachandran & Sasisekharan, 1968; Scheraga, 1968; Venkatachalam & Ramachandran, 1969; Scheraga, 1971; Brant, 1972; Hopflnger, 1973). A critical evaluation of these techniques as applied to the polynucleotides has recently appeared (Olson, 1975 a, b).