On page 94 of Ignition! an equation for "Frozen equilibrium calculation":
$$ c = \left[ 2H/M \right]^{1/2} \left[ 1- \left( {P_e \over P_c} \right) ^{R/C_p} \right]^{1/2} $$
All quantities are explained, except for $C_p$ - it only appears in the sentence "$\gamma$ is the ratio of specific heats, $C_p/C_v$ of the chamber gases" in a prior equation, but without explaining which of these two is specific heat of what. Then it's later referenced frequently, being an essential parameter, but always only by the $C_p$ symbol.
This is baffling especially in light of the page 99 quote: "As for the solids, $C$, $Al_2 O_3$,and $B_2 O_3$, their $R/C_p$ is precisely zero." Since $R$ is a constant, that would imply infinite $C_p$ - an infinite specific heat? what the heck? What is that quantity?