Marsh and Williamson had left Appenoose's village at Ottumwa and traveled two days downriver on horseback to the trading post of William Phelps (see below).
laudanum: a tincture of opium dissolved in alcohol, widely used in 19th-century homes to treat a number of different symptoms.
Phelps: In his report to the American Board the following year, Marsh described William Phelps this way: “although a professed infidel in sentiment, [Phelps] still was friendly to my object, treated me with great hospitality and would take no pay for my board whilst I staid. He and a brother of his also are trading in opposition to the Am[erican] F[ur] Co. and it rather operated to our advantage than otherwise. And Mr. P. declared that if something was not done soon for the Sacs &c. they would all be swept off.” Wisconsin Historical Collections 15: 154]
According to Henry Sabin’s The Making of Iowa (N.Y., 1900), “Captain Phelps was so jolly that the Indians termed him Che-che-pe-qua, or 'Winking Eyes.'"
Phelps trading post: According to the City of Oquawka, Ill., home page, "S. S. Phelps was accompanied by a younger brother, William, and for many years the lives of the two brothers were inseparable... [William] established trading stations on the Des Moines River where Farmington and Ottumwa, Iowa, now stand. William was the husband of Carolina Phelps who wrote a diary of her experiences in the wilderness...
"The brothers soon established an extensive trade with the Sac and Fox Indians. William went [first, ca. 1828] to Upper Yellow Banks, New Boston, and succeeded in holding a position opposite the mouth of the Iowa River for the purpose of trading with Chief Keokuk’s people who had a village on the Iowa River a few miles above where it empties into the Mississippi River. After remaining there until their trade with those Indians was secured, William returned to Oquawka.
"About this same time  Dr. Galland had become discouraged with his newest location ["a few miles down river... near Montrose, Iowa", above the mouth of the Des Moines] and sent word to the Phelps Brothers that if they could hold his claim there they were welcome to it. William moved down there and after several skirmishes with both red and white man, held his position and thus had access to the Indian villages of a large scope of country."
When Marsh visited in 1834, therfore, it's likely that Phelps was located in the vicinity of modern Montrose and Keokuk, Iowa (map).
The American Fur Co. had been organized by New York merchant John Jacob Astor in 1808; it was the dominant trading company on the frontier at this time, and the chief rival of the British-controlled Northwest and Hudson Bay companies in the north until the 1840's.
Williamson’s farewell: Williamson, like Marsh, had been asked that spring to see if western tribes would appreciate having a missionary. He ascended the Mississippi to modern St. Paul (then the highest white settlement), and visited Prairie du Chien and Rock Island to collected information about the Sioux. He headed back to Ripley, Ohio, in July 1834, was ordained a minister in September, and later became a missionary in what is now Minnesota, where he died in 1879.