GENERAL ASPECTS OF UPPER ATMOSPHERIC PHYSICS 
mechanical method of the collisional cross sections of 
these molecules and atoms is also necessary. It may be 
recalled that ionospheric studies yield collisional fre- 
quencies of electrons with atmospheric particles, and 
one is tempted to infer from this the molecular densities 
in the respective regions. However, this is not justi- 
fiable since the cross sections of the atoms and mole- 
cules for low-velocity colliding electrons (corresponding 
to a temperature, for instance, of 1000K—a few tenths 
of an electron volt) may be widely different from the 
gas kinetic cross section [76, 94]. More theoretical and 
experimental data on the collisional processes between 
meteor atoms and air molecules are also needed. This 
will be helpful in improving the theoretical calculations 
on meteors. 
Tt is a pleasure to record my thanks to Dr. 8. N. 
Ghosh, Imperial Chemical Industries Research Fellow 
of the National Institute of Sciences of India, now 
working in my laboratory, for the help he has given 
me in the preparation of this article by collecting data, 
checking references, and suggesting improvements. 
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