What is the test for establishing a 'rule of law' in the context of Charter amendments?

In Commonwealth v. Essex Co. 13 Gray,(Mass.) 239-253, Mr. Justice Shaw says: 'It seems to us that this power must have some limit, though it is difficult to define it. * * * Perhaps from these extreme cases-- for extreme case are allowable to test a legal principle-- the rule to be extracted is this: that where, under a power in a charter, rights have been acquired and become vested, no amendment or alteration of the charter can take away the property or rights which have become vested under a legitimate exercise of the powers granted. ' Page 253.

'This rule,' says Mr. Justice Strong, 'has been recognized ever since.' 99 U.S. 700-742.