Out of this peace and concord suddenly sprang up, as Jefferson said, "like a fire-bell in the night," a question which had silently divided the Union, and threatened to dissolve it. It was the question of slavery. During the whole course of the Napoleonic wars the country had been occupied in the defence of its neutral trade; since 1815 it had been busy in reorganizing its commercial and political system. During this time, however, four new States had been admitted into the Union: of these, two--Ohio and Indiana-- came in with constitutions prohibiting slavery; two--Louisiana and Mississippi--had slaves. This balance was not accidental; it was arranged so as to preserve a like balance in the Senate. [Sidenote: Slavery profitable.] [Sidenote: Slave-trade forbidden.] The movement against slavery had by no means spent itself: there were still emancipation societies both North and South. In 1794 Jay appeared to suppose that cotton was not an American export (§ 85); but since the invention of the cotton-gin in 1793 the cultivation of cotton by slave labor had grown more and more profitable, and in 1820 that export was valued at nearly twenty millions. The planters of the northern belt of slaveholding States did not share in this culture, but they found an increasing sale for their surplus blacks to their Southern neighbors; they had, therefore, joined with members from the Northern States in the act of March 2, 1807, to prohibit the importation of slaves. The act was insufficient, inasmuch as the punishment provided was slight, and slaves captured while in course of illegal importation were sold for the benefit of the States into which they were brought, In 1820 the slave-trade was made piracy, so that the nominal penalty was death. [Sidenote: Schemes of colonization.] One evidence of the uneasiness of the country on the slavery question was the formation of the American Colonization Society in 1816. Its purpose was to encourage emancipation, and thus to reduce the evils of slavery, by drawing off the free blacks and colonizing them in Africa. It had a large membership throughout the country; James Madison and Henry Clay were among its presidents. Some States made grants of money in its aid, and after 1819 the United States assisted it by sending to the African colony slaves captured while in course of illegal importation. The whole scheme was but a palliative, and in fact rather tended to strengthen slavery, by taking away the disquieting presence of free blacks among the slaves. The Society, however, never had the means to draw away enough negroes sensibly to affect the problem; the number which they exported was replaced many times over by illegal importations from Africa. [Sidenote: Fugitive slaves.] [Sidenote: District of Columbia.]