This article studies the circumstances under which Baring's Bank, in London, became involved in Spanish finance in the middle of the nineteenth century, and the struggle between these bankers and the Rothschilds, traditional bankers of the government, over the mercury of Almadén. Baring's did not lack the support of some members of the government, including Bravo Murillo, who attempted in vain to break the links of dependency established with the Rothschilds since 1834. Using documents from both banks in London, the article presents information about the financial world of Madrid in the mid-nineteenth century, especially concerning organizations so little known as the Banco de Ultramar and Fomento, as well as on various individual bankers like O'Shea, agents of Baring's in Madrid.