5. That, by the side of his international law, with its basis in natural law, there was also a positive international law, was not unrecognized by Grotius, but his purpose was merely to depict a system of international law which should compel universal observance irrespective of time and nation. And shortly after Grotius, Zouche and his followers did indeed attempt, in opposition to him, to formulate just such a positive international law, but it could not win for itself, at any rate in the seventeenth century, any great recognition; development was overshadowed by the system of Grotius, and many of his rules of natural law gradually obtained recognition in practice as customary law. But the increasing intercourse of states in the eighteenth century called forth a more positive school of international jurists, and the works of Bynkershoek, Moser, and Martens fertilized the soil on which in the nineteenth century there could gradually grow a really positive theory of international law, even if the scales which betoken its past connexion with natural law still adhere to the international law of to-day. [Sidenote: International legislation initiated by the Congress of Vienna.] 6. A positive theory of international law was demanded by the fact that in the first quarter of the nineteenth century, with the Final Act of the Congress of Vienna, the quasi-legislative activity of international conventions asserted itself for the first time. From then onwards, general international law was frequently evolved by means of an international convention. It was in this way that the permanent neutralization of Switzerland, Belgium, and Luxemburg was effected, the navigation of the so-called international rivers in Europe declared free, the slave-trade abolished, the grades of diplomatic agents regulated, privateering abolished, the necessity of effectiveness in a blockade recognized, the principle 'free ships, free goods' finally established, neutral goods on enemy ships declared free, rules provided in the interest of those wounded in battle, explosive bullets under the weight of 400 grammes forbidden, the Suez Canal neutralized, and so forth. [Sidenote: International Administrative Union.] 7. Another fact of great importance is the endeavour, which first manifested itself in the World Postal Union of 1874, to carry out the international administration of common interests, economic and other, by means of more or less general international unions. In this way a series of international administrative unions, often conjoined with special international boards, have been called into existence. [Sidenote: Legislation of the Peace Conferences and of the Naval Conference of London.]