SIR,--Our country is opening up a new page in the history of governments. The world has never witnessed such a spontaneous uprising of any people in support of free institutions as that now exhibited by the citizens of our Northern States. I observe that the vexed question of slavery still has to be met, both in the Cabinet and in the field. It has been met by former Presidents, by former Cabinets, and by former military officers. They have established a train of precedents that may be well followed at this day. I write now for the purpose of inviting attention to those principles of international law which are regarded by publicists and jurists as proper guides in the exercise of that despotic and almost unlimited authority called the "war power." A synopsis of these doctrines was given by Major General Gaines, at New Orleans, in 1838. General Jessup had captured many fugitive slaves and Indians in Florida, and had ordered them to be sent west of the Mississippi. At New Orleans, they were claimed by the owners, under legal process; but Gen. Gaines, commanding that military district, refused to deliver them to the sheriff, and appeared in court, stating his own defence. He declared that these people (men, women and children) were captured in wars and held as prisoners of war: that as commander of that military department or district, he held them subject only to the order of the National Executive: that he could recognize no other power in time of war, or by the laws of war, as authorized to take prisoners from his possession. He asserted that, in time of war, all slaves were belligerents as much as their masters. The slave men, said he, cultivate the earth and supply provisions. The women cook the food, nurse the wounded and sick, and contribute to the maintenance of the war, often more than the same number of males. The slave children equally contribute whatever they are able to the support of the war. Indeed, he well supported General Butler's declaration, that slaves are contraband of war. The military officer, said he, can enter into no judicial examination of the claim of one man to the bone and muscle of another as property. Nor could he, as a military officer, know what the laws of Florida were while engaged in maintaining the Federal Government by force of arms. In such case, he could only be guided by the laws of war; and whatever may be the laws of any State, they must yield to the safety of the Federal Government. This defence of General Gaines may be found in House Document No. 225, of the Second Session of the 25th Congress. He sent the slaves West, where they became free.