Classes of cases under the clause 339 Public grants 339 Municipal corporations 339 Public offices 340 Revocable privileges versus "contracts": Tax exemptions 341 Vested rights 343 Reservation of the right to alter and repeal 343 Right to reserve: When limited 343 Corporations as persons subject to the law 345 Corporations and the police power 345 Strict construction of public grants 346 Charles River Bridge case 346 Application of the strict construction rule 346 Strict construction of tax exemptions 347 Strict construction and the police power 348 Doctrine of inalienable State powers 349 Eminent domain power inalienable 349 Taxing power not inalienable 350 Police power: When inalienable 351 Private contracts 352 Scope of the term 352 Source of the obligation 352 Ogden _v._ Saunders 353 Remedy a part of the obligation 354 Establishment of the rule 354 Qualifications of the rule 355 Municipal Bond cases 356 Private contracts and the police power 357 Emergency legislation 358 Individual rights versus public welfare 359 Evaluation of the clause today 359 Statistical data pertinent to the clause 361 Clause 2. Not to levy duties on exports and imports 362 Duties on exports and imports 362 Scope 362 Privilege taxes 363 Property taxes 364 Inspection laws 364 Clause 3. Not to lay tonnage duties, keep troops, make compacts, or engage in war 365 Tonnage duties 365 Keeping troops 366 Interstate compacts 366