Wilmot Proviso
2 of the 7 encyclopedias on this shelf carry an entry for Wilmot Proviso. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
a notable resolution introduced into the United States Congress by David Wilmot. On Aug. 8, 1846, pending the consideration in Congress of a bill placing $2,000,000 at the disposal of President Polk to negotiate a peace with Mexico, David Wilmot, a representative from Pennsylvania, offered the following amendment: "Provided that, as an express and fundamental condition to the acquisition of any territory from the republic of Mexico by the United States, by virtue of any treaty which may be negotiated between them, and to the use by the Executive of the moneys therein appropriated, neither slavery nor involuntary servitude shall ever exist in any part of said territory, except for crime, whereof the party shall be duly convicted." This was the famous "Wilmot Proviso," which became the source of great agitation throughout the country. It was adopted in the House by a vote of 94 to 78, and was under debate in the Senate when the hour arrived previously fixed for the adjournment of the session. At the next session, Mr. Wilmot again introduced it, and a fierce and angry contest commenced. The House remained firm in favor of the amendment, and it was passed by a decided majority, but not acted on by the Senate.
Aiton's Encyclopedia (1910)
in American history, a proposed restriction upon the extension of slavery. August 8, 1846, President Polk sent a special message to Congress requesting an appropriation for "the adjustment of a boundary with Mexico," the purpose being to purchase territory not acquired with Texas. A bill appropriating $2,000,000 was introduced in the House. Daniel Wilmot, a Pennsylvania Democrat, offered an amendment, since known as the "Wilmot Proviso," providing that "neither slavery nor involuntary servitude shall ever exist in any part of said territory except for crime whereof the party shall first be duly convicted." The bill as amended passed the House, but it was stopped in the Senate. At the next session of Congress, a bill was proposed appropriating $3,000,000. Wilmot offered his amendment a second time, but after an angry debate the bill was passed without the amendment. The object of the proviso was not attained until 1862 when Congress passed an act prohibiting slavery "in any of the territories of the United States now existing or which may at any time hereafter be acquired."