An exaction, called a tax, which is in fact and effect a penalty, is not a tax. While the lawmaker is entirely free to ignore the ordinary meaning of words and make definitions of his own, that device may not be employed so as to change the nature of acts or things to which the words are applied. The Constitution made no grant of authority to Congress to legislate substantively for the general welfare, and no such authority exists, save as the general welfare may be promoted by the exercise of the powers which are granted. Cases supra . The power of taxation which is expressly granted may be adopted as a means to carry into operation another power also expressly granted, but resort to the taxing power to effectuate an end which is not legitimate, not within the scope of the Constitution, is obviously inadmissible.
Cases supra .
If the Constitution, in its grant of powers, is to be so construed as to carry into full effect the power granted, it is equally imperative that, where a prohibition or limitation is placed upon the power of Congress, that prohibition or limitation should be enforced in its spirit and to its entirety. While it is true that, where the law merely imposes the tax without disclosing the indirect purpose of its imposition, the courts might hesitate to declare the law unconstitutional, on the other hand, if the real purpose of the law is disclosed on its face to be a purpose that invades the police powers reserved to the individual States, the courts should not hesitate to declare the Act an unconstitutional usurpation by the Federal Government of powers reserved to the States by the Tenth Amendment. Under the American constitutional system, the police power, being an attribute of sovereignty inherent in the original States, and not delegated by the Federal Constitution to the United States, remains with the individual States. Whatever may be the motive or pretext of a statute, or in whatever language it may be framed, its real purpose and the question of its validity must be determined by its natural and reasonable effect to be ascertained from its practical operation. It is apparent from reading the National Firearms Act that Congress had no intention of framing a law that would procure any revenue for the Government.