Berzelius was a prolific correspondent with leading scientists of his time, such as Gerardus Johannes Mulder, Claude Louis Berthollet, Humphry Davy, Friedrich Wöhler, Eilhard Mitscherlich and Christian Friedrich Schönbein.
In 1812, Berzelius traveled to London, England, including Greenwich to meet with prominent British scientists of the time. These included Humphry Davy, chemist William Wollaston, physician-scientist Thomas Young, astronomer William Herschel, chemist Smithson Tennant, and inventor James Watt, among others. Berzelius also visited Davys laboratory. After his visit to Davys laboratory, Berzelius remarked, "A tidy laboratory is a sign of a lazy chemist."
Humphry Davy in 1810 proposed that chlorine is an element. Berzelius rejected this claim because of his belief that all acids were based on oxygen. Since chlorine forms a strong acid (muriatic acid, modern HCl), chlorine must contain oxygen and thus cannot be an element. However, in 1812, Bernard Courtois proved that  iodine is an element. Then in 1816 Joseph-Louis Gay-Lussac demonstrated that prussic acid (hydrogen cyanide) contains only hydrogen, carbon, and nitrogen, and no oxygen. These findings persuaded Berzelius that not all acids contain oxygen, and that Davy and Gay-Lussac were correct: chlorine and iodine are indeed elements.