  This step was again triggered by a paper of Maxwell, this time one that must have pleased Boltzmann very much, since it was called “On Boltzmann's theorem” (Maxwell 1879) and dealt with the theorem discussed in the last section of his (1868). He pointed out that this theorem does not rely on any collision assumption. But Maxwell also made some pertinent observations along the way. He is critical about Boltzmann's ergodic hypothesis, pointing out that “it is manifest that there are cases in which this does not take place” (Maxwell 1879, 694). Apparently, Maxwell had not noticed that Boltzmann's later papers had also expressed similar doubts. He rejected Boltzmann'a time-average view of probability and instead preferred to interpret ρ as an ensemble density. Further, he states that any claim that the distribution function obtained was the unique stationary distribution “remained to be investigated” (Maxwell 1879, 722). Maxwell's paper seems to have revived Boltzmann's interest in the ergodic hypothesis, which he had been avoiding for a decade. This renewed confidence is expressed, for example in Boltzmann (1887):