With reference, also, to various matters which, properly speaking, cannot be said to fall within the domain of the police power, the state legislatures have been held to be incompetent to contract.

42 The court say: "We do not question the existence of a state power to levy taxes as claimed, nor the subordination of contracts to it, so far as it is unrestrained by constitutional limitation. But the power is not without limits, and one of its limitations is found in the clause of the federal Constitution, that no State shall pass a law impairing the obligation of contracts. A change of the expressed stipulations of a contract, or a relief of a debtor from strict and literal compliance with its requirements, can no more be affected by an exertion of the taxing power than it can be by the exertion of any power of a state,legislature. The constitutional provision against impairing contract obligations is a limit upon the taxing power, as well as upon all legislation, whatever form it may assume. Indeed, attempted state taxation is the mode most frequently adopted to affect contracts contrary to the constitutional inhibition. It most frequently calls for the exercise of our supervisory power. It may, then, safely be affirmed that no State, by virtue of its taxing power, can say to a debtor, 'You need not pay to your creditor all of what you have promised to him. You may satisfy your duty by retaining a part for yourself, or for some municipality, or for the state treasury.' Much less can a city say, 'We will tax our debt to you. and in virtue of the tax withhold a part for our use.' ... Is, then, property which consists in the promise of a State, or of a municipality of a State, beyond the reach of taxation? We do not affirm that it is. A State may undoubtedly tax any of its creditors within its jurisdiction for the debt due to him, and regulate the amount of the tax by the rate of interest the debt bears, if its promise be left unchanged. A tax thus laid impairs no obligation assumed. It leaves the contract untouched. But until payment of the debt or interest has been made, as stipulated, we think no act of state sovereignty can work an exoneration from what has been promised to the creditor, namely: payment to him without a violation of the Constitution. 'The true rule of every case of property founded on contract with the government is this: it must first be reduced into possession, and then it will become subject, in common with other similar property, to the right of the government to raise contributions upon it. It may be said that the government may fulfill this principle by paying the interest with one hand, and taking back the amount of the tax with the other. But to this the answer is, that, to comply truly with the rule, the tax must be upon all the money of the community, not upon the particular portion of it which is paid to the public creditors, and it ought besides to be so regulated as not to include a lien of the tax upon the fund.

Thus in Newton v. Commissioner43 it was declared, with reference to the location of a county seat, that one legislature could not bind its successors. So, also, in Illinois Central R. R. Co. v. Illinois44 the Supreme Court held that the people of the State were, as a continuing whole, interested in the navigable waters of the State and in the lands under them, and that, therefore, the title to them was held in trust by the State and could not be ceded away.

In Munn v. Illinois45 and in the Granger Cases,46 the doctrine of the regulative power of the States over public service corporations, and those whose business is affected with a public interest, was established, and that this is a power the exercise of which is not to be construed as restrained by charter provisions except when it plainly appears that this has been intended. And, even when the grant is in unequivocal language, it will not be held valid against subsequent legislation as to matters which vitally or even seriously affect the public welfare, that is, relate to subjects within the field of legitimate police control. In this respect the protection of private rights under the due process clause and under the obligation clause is the same.