= Normal hydrogen
{title2=$x_{\rm o}=3/4,\quad x_{\rm p}=1/4$}
<Normal hydrogen> denotes the $3:1$ <orthohydrogen>-to-<parahydrogen> mixture, the high-<temperature> equilibrium limit. Near room <temperature> the equilibrium ratio is already close to this limit. Cooling an isolated sample need not change its composition on the measurement timescale: ordinary collisions can redistribute rotational populations within each spin species much faster than they interconvert the spin species. A kinetically frozen composition can therefore coexist with rotational <thermal equilibrium> separately within each species.
The distinction was historically decisive. The institutional account https://www.mpinat.mpg.de/607569/para-Wasserstoff[Die Entdeckung des para-Wasserstoffs], in its discussion of the rotational <heat capacity>, explains that assuming an equilibrating composition missed the observed behavior, whereas freezing the high-<temperature> ratio reproduced it. It also records <Karl Friedrich Bonhoeffer> and <Paul Harteck>'s 1929 preparation using a <catalyst> and <thermal conductivity> detection.<Die Entdeckung des para-Wasserstoffs>[paragraphs on frozen high-temperature composition and the 1929 charcoal/thermal-conductivity discovery]{ref}
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