(_b_) The decisions of =domestic courts= upon such matters as extradition,[47] diplomatic privileges, piracy, etc., tend to become a source of international law. In the United States the Supreme Court has original jurisdiction "in all cases affecting ambassadors, other public ministers, and consuls."[48]
(_c_) The decisions of =courts of arbitration= and other mixed courts are usually upon broad principles. Some of the principles involved may become established precedents, yet the tendency to render a decision, which by a compromise may be measurably acceptable to both parties, may lessen the value of the decision as a precedent. As arbitration is of necessity voluntary, there is generally a consensus upon certain points, even though the decision rendered may not become a precedent. The growth of the practice of arbitration of disputes is an indication of the general recognition of mutual confidence between states. The principles upon which the court of arbitration bases its decision, rather than the decision itself, furnish material valuable for international law.
Treaties and state papers of whatever form[49] indicate the state of opinion, at a given time, in regard to the matters of which they speak. Since they are binding upon the parties to them, treaties may be regarded as evidence of what the states, bound by their terms, accept as law. When the same terms are generally accepted among nations, treaties become a valuable evidence of concrete facts of practice and proper sources of international law. The principles may be so well established by successive treaties as to need no further treaty specification. Treaties and state papers vary greatly in value as sources of international law, however.
(_a_) Treaties and state papers may lay down new rules or outline the operation of old rules. As instances of those laying down new rules may be taken the Clayton-Bulwer Treaty of April 19, 1850, the convention for the protection of Submarine Cables, March 14, 1884, the Geneva Convention of 1864; of those outlining and determining the operation of old rules, there are many instances, the most numerous in the treaties in regard to maritime affairs and consuls.
(_b_) Treaties and state papers may enunciate established rules as understood by the parties to the treaty. The Declaration of the Conference of London, Jan. 17, 1871, to which the major European states were parties, announces that the signatory powers "recognize that it is an essential principle of the Law of Nations that no Power can liberate itself from the engagements of a Treaty, nor modify the stipulations thereof, unless with the consent of the Contracting Powers by means of an amicable agreement."[50]