Encyclopaedia Britannica, 11th Edition, "Gassendi, Pierre" to "Geocentric" Volume 11, Slice 5
i. Military hospital-ships, that is to say, ships constructed or
assigned by states specially and solely for the purpose of assisting the wounded, sick or shipwrecked, and the names of which shall have been communicated to the belligerent powers at the commencement or during the course of hostilities, and in any case before they are employed, shall be respected and cannot be captured while hostilities last.
These ships, moreover, are not on the same footing as men-of-war as regards their stay in a neutral port.