 Humboldt had also hinted at his knowledge of New Spain.Arriving in Philadelphia, which was a center of learning in the U.S., Humboldt met with some of the major scientific figures of the era, including chemist and anatomist Caspar Wistar, who pushed for compulsory smallpox vaccination, and botanist Benjamin Smith Barton, as well as physician Benjamin Rush, a signer of the Declaration of Independence, who wished to hear about cinchona bark from a South American tree, which cured fevers.Humboldt later published a treatise in English on cinchona.