forbidden regulations

Such are the taxes already noted which are aimed at or discriminate against the commerce or impose a levy for the privilege of doing it, or tax interstate transportation or communication or their gross earnings, or levy an exaction on merchandise in the course of its interstate journey. Each imposes a burden which intrastate commerce does not bear, and merely because interstate commerce is being done places it at a disadvantage in comparison with intrastate business or property in circumstances such that if the asserted power to tax were sustained, the states would be left free to exert it to the detriment of the national commerce.