This Declaration gives expression, with regard to a particular instrument of warfare, to the customary rules prohibiting the use of poison and of material causing unnecessary suffering (cf. Oppenheim/Lauterpacht, International Law, 7th ed., Vol.2, p.342). These customary rules were formally enacted in Articles 23(a) and 23(e) of the Hague Regulations of 1899 and 1907. The prohibition of the use of poison is also mentioned in Article 70 of the Lieber Instructions, Article 13(a) of the Brussels Declaration of 1874 and Article 8(a) of the Oxford Manual adopted by the Institute of International Law in 1880. For more recent conventions concerning the use of gases, see Treaty of Washington of 1922, the Geneva Protocol of 1925 and Resolution 2603 (XXIV) of 1969 of the United Nations General Assembly.