Before the time arrived for the Utica Convention to assemble, the anti-slavery revolt was widely extended, and was, apparently, no less against Taylor than against Cass. There was agitation in many States, and the Barnburners found that by uniting with the opposition against both the old parties, a most effective combination could be made. It was certain to profit them in New York, and it promised the special revenge which they desired in the defeat of Cass. The various local and State movements were merged in one great convention, which met at Buffalo on the 9th of August, with imposing demonstrations. Many of those composing it had held high rank in the old parties. Salmon P. Chase of Ohio was selected as president. The convention represented a genuine anti-slavery sentiment, and amid excitement and enthusiasm Martin Van Buren was nominated for President, and Charles Francis Adams for Vice-President. The Barnburners, the anti-slavery Whigs, and the old Abolitionists, co-operated with apparent harmony under the general name of the Free-soil party; and the impression with many when the convention adjourned was, that Mr. Van Buren would have a plurality over both Cass and Taylor in the State of New York. The management of the popular canvass was intrusted to Democratic partisans of the Silas Wright school, and this fact had a significant and unexpected influence upon the minds of anti-slavery Whigs. In the first flush of the excitement, the supporters of the regular Democratic nominee were not alarmed. They argued, not illogically, that the Free-soil ticket would draw more largely from the Whigs than from the Democrats, and thus very probably injure Taylor more than Cass. But in a few weeks this hope was dispelled. The Whigs of the country had been engaged for a long period in an earnest political warfare against Mr. Van Buren. In New York the contest had been personal and acrimonious to the last degree, and ordinary human nature could hardly be expected the bury at once the grievances and resentments of a generation. Nor did the Whigs confide in the sincerity of Mr. Van Buren's anti-slavery conversion. His repentance was late, and even the most charitable suspected that his desire to punish Cass had entered largely into the motives which suddenly aroused him to the evils of slavery after forty years of quiet acquiescence in all the demands of the South. Mr. Seward, who possessed the unbounded confidence of the anti-slavery men of New York, led a most earnest canvass in favor of General Taylor, and was especially successful in influencing Whigs against Van Buren. In this he was aided by the organizing skill of Thurlow Weed, and by the editorial power of Horace Greeley. Perhaps in no other National election did three men so completely control the result. They gave the vote of New York to General Taylor, and made him President of the United States. MR. WEBSTER'S MARSHFIELD SPEECH.