The thermodynamics of the Hagedorn mass spectrum
  No bootstrap assumption is needed to derive the exponential growth of the Hagedorn hadron mass spectrum: It is a consequence of the second law applied to a relativistic gas, and the relativistic equivalence between inertial mass and its heat content. The Hagedorn temperature occurs in the limit as the number of particles and their internal energy diverge such that their ratio remains constant. The divergences in the $N$ particle entropy, energy, and free energy result when this condition is imposed upon a mixture of ideal gases, one conserving particle number and the other not. The analogy with a droplet in the presence of vapor explains why the pressure of the droplet continues to increase as the temperature rises finally leading to its break up when the Hagedorn temperature is reached. The adiabatic condition relating the particle volume to the Hagedorn temperature is asymptotic. Since it is a limiting temperature, and not a critical one, there can be no phase transition of whatever kind, and the original density of states used to derive such a phase transition is not thermodynamically admissible because its partition function does not exist. 
B. H. Lavenda