The 21-cm absorption by a neutral hydrogen gas between the galaxies has been searched for by means of observations of the extragalactic radio source Cygnus A. The negative results yield an upper limit on the opaciry of 0.0075. This result implies that if a homogeneous distribution of neutral hydrogen is expanding with the universe, it has a density-temperature ratio less than 8.1 X 10- . The temperature referred to is the excitation temperature of the hydrogen hyperfine levels; it is shown in the following paper (Field 1959) that it is not simply equal to the gas kinetic temperature but can be calculated theoretically.
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George B. Field (1959) studied this question.