All lands granted by Congress to this state for educational purposes, all estates that escheat to the state, all property given or bequeathed to the state for educational purposes, and the proceeds derived from these sources, together with that percentage of the proceeds from the sale of federal lands which has been granted by Congress to this state without restriction or for educational purposes and all fines collected under the penal laws of the state are hereby pledged for educational purposes and the money therefrom must not be transferred to other funds for other uses. The interest only earned on the money derived from these sources must be apportioned by the legislature among the several counties for educational purposes, and, if necessary, a portion of that interest may be appropriated for the support of the state university, but any of that interest which is unexpended at the end of any year must be added to the principal sum pledged for educational purposes.