In addition to the lands heretofore granted to the University of Texas, there is hereby set apart, and appropriated, for the endowment maintenance, and support of said University and its branches, one million acres of the unappropriated public domain of the State, to be designated, and surveyed as may be provided by law; and said lands shall be sold under the same regulations, and the proceeds invested in the same manner, as is provided for the sale and investment of the permanent University Fund; and the Legislature shall not have power to grant any relief to the purchasers of said lands.