  However, his doubts were still not laid to rest. His next paper on gas theory (1871a) returns to the study of a detailed mechanical gas model, this time consisting of polyatomic molecules, and explicitly avoids any reliance on the ergodic hypothesis. And when he did return to the ergodic hypothesis in (1871b), it was with much more caution. Indeed, it is here that he actually first described the worrying assumption as an hypothesis, formulated as follows: