tate "may exempt
particular parcels of property or the property of particular persons or
corporations from taxation, either for a specified period or
perpetually, or may limit the amount or rate of taxation, to which such
property shall be subjected," and such an exemption is frequently a
contract within the sense of the Constitution. Indeed this is always so
when the immunity is conferred upon a corporation by the clear terms of
its charter.[1641] When, on the other hand, an immunity of this sort
springs from general law, its precise nature is more open to doubt, as a
comparison of decisions will serve to illustrate.

In Piqua Branch of the State Bank _v._ Knoop,[1642] a closely divided
Court held that a general banking law of the State of Ohio which
provided that companies complying therewith and their stockholders
should be exempt from all but certain taxes, was, as to a bank organized
under it and its stockholders, a contract within the meaning of article
I, section 10. "The provision was not," the Court said, "a legislative
command nor a rule of taxation until changed, but a contract stipulating
against any change, from the nature of the language used and the
circumstances under which it was adopted."[1643] When, however, the
State of Michigan pledged itself, by a general legislative act, not to
tax any corporation, company, or individual undertaking to manufacture
salt in the State from water there obtained by boring on property used
for this purpose and, furthermore, to pay a bounty on the salt so
manufactured, it was held not to have engaged itself within the
constitutional sense. "General encouragements," said the Court, "held
out to all persons indiscriminately, to engage in a particular trade or
manufacture, whether such encouragement be in the shape of bounties or
drawbacks, or other advantage, are always under the legislative control,
and may be discontinued at any time."[1644] So far as exemption from
taxation is concerned the difference between these two cases is
obviously slight; but the later one is unquestionable authority for the
proposition that legislative bounties are repealable at will.

Furthermore, exemptions from taxation have in certain cases been treated
as gratuities repealable at will, even when conferred by specific
legislative enactments. This would seem always to be the case when the
beneficiaries were already in existence when the exemption was created
and did nothing of a more positiv