64 AN INTRODUCTION TO THE STUDY OF VIRUSES 
virus, i.e. the volume of that part of the virus within which energy 
must be absorbed from the radiation for inactivation to occur. Ab- 
sorption is a highly localized phenomenon, and is sufficiently energetic 
for it to be tolerably certain that when energy absorption, i.e. ioniza- 
tion, occurs in a particular atom, the molecule or radicle of which 
that atom is a part suffers chemical change. In a large virus like that 
of vaccinia, the radio-sensitive volume is only a very small fraction 
of the volume of the virus. On the other hand, a virus which crystal- 
lizes is presumably a molecular species and it seems legitimate to 
conclude that the radio-sensitive volume will be identical with the 
volume of one molecule of the virus. For further information on 
these methods of measurement of virus-particle size, the reader is 
referred to a monograph by Markham, Smith, and Lea (1942). 
A mote recent method of measurement has been described by Oster 
(1947); experiments have shown that the size and shape of high 
polymeric molecules, colloidal particles, and viruses can be determined 
by measuring the light scattered by solutions of these substances at 
different angles to the incident light. 
