In Arizona Grocery Co. v. Atchison, Topeka & Santa Fe Ry. Co., 284 U.S. 370, 52 S.Ct. 183, 186, 76 L.Ed. 348 (1932), it was held that where the Interstate Commerce Commission had, upon complaint and after hearing, declared what was the maximum reasonable rate to be charged by a carrier, it may not, at a later time, and upon the same or additional evidence as to the fact situation existing when its previous order was promulgated, subject a carrier which conformed thereto to the payment of reparation measured by what the Commission then held it should have decided in the earlier proceeding, by declaring its own prior finding as to reasonableness to be erroneous. In passing upon the case, the court said: 'The Commission's error arose from a failure to recognize that, when it prescribed a maximum reasonable rate for the future, it was performing a legislative function, and that, when it was sitting to award reparation, it was sitting for a purpose judicial in its nature. In the second capacity, while not bound by the rule of res judicata, it was bound to recognize the validity of the rule of conduct prescribed by it, and not to repeal its own enactment with retroactive effect. It could repeal the order as it affected future action, and substitute a new rule of conduct as often as occasion might require, but this was obviously the limit of its power, as of that of the Legislature itself.'