In our view the necessary effect of this act is, by means of a prohibition against the movement in interstate commerce of ordinary commercial commodities, to regulate the hours of labor of children in factories and mines within the states, a purely state authority. Thus the act in a two-fold sense is repugnant to the Constitution. It not only transcends the authority delegated to Congress over commerce but also exerts a power as to a purely local matter to which the federal authority does not extend.