The fields of plasma physics, astrophysics, and all of chemistry use absorption and scattering of optical radiation as primary diagnostic methods. Volumes 9 and 12 of this series present a great deal of material relevant to the measurement of the existence of matter by beam absorption and scattering, and Part 6 of this volume furnishes guidance to the experimentalist seeking to learn of methods for measurement of the composition of a fluid in motion. Usually consideration of the interaction of the different species making up the fluid with the beam of radiation is necessary in order to interpret measurements to yield the total density. This becomes less true as the radiation considered becomes "harder," i.e., for x rays, gamma rays, mesons, and neutrinos.