§ 65. The surplus number of prisoners of war remaining after an exchange has taken place, is sometimes released either for the payment of a stipulated sum of money, or, in urgent cases, of provision, clothing, or other necessaries. Such arrangement, however, requires the sanction of the highest authority. § 66. The exchange of prisoners of war is an act of convenience to both belligerents. If no general cartel has been concluded, it cannot be demanded by either of them. No belligerent is obliged to exchange prisoners of war. A cartel is null and void so soon as either party has violated it. § 67. No exchange of prisoners shall be made except after complete capture, and after an accurate account of all, and a list of the captured officers, has been taken. No exchange shall take place during or immediately after an engagement. § 68. A flag of truce cannot insist on being admitted. It must always be admitted with great caution. Unnecessary frequency is carefully to be avoided. A flag of truce offering himself during an engagement can be admitted as a very rare exception only. It is no breach of good faith to retain such a flag of truce, if admitted during the engagement. Firing is not allowed to cease at the appearance of a flag of truce in battle. If a flag of truce, presenting himself during an engagement, is killed or wounded, it furnishes no ground of complaint whatever. § 69. It is customary to designate by certain flags of protection the hospitals, in places which are shelled, so that the besieging enemy may avoid firing on them. The same has been done in battles, when hospitals are situated within the district of the engagement. An honorable belligerent allows himself to be guided by these flags or signals of protection as much as the contingencies and the necessities of the fight will permit. Honorable belligerents even request by flags of truce to designate the hospitals within the territory of the enemy, so that they may be spared. It is duly considered an act of military bad faith, of infamy or fiendishness, to deceive the enemy either by such flags of protection, or by the request to hoist them. § 70. The besieging belligerent has sometimes requested the besieged to designate the buildings containing collections of works of art, scientific museums, astronomical observatories or precious libraries, so that their destruction may be prevented as much as possible. The United States highly commend such conduct to their armies, and remind them that some instances of this care for civilization in the midst of destructive war, even in remote antiquity, are recorded in history. THE PAROLE. § 71. Prisoners of war may be released from captivity not only by exchange, but, under certain circumstances, also by parole.