The earliest authentic works on European alchemy are those of the English monk Roger Bacon and the German philosopher St. Albertus Magnus. In their treatises they maintained that gold was the perfect metal and that inferior metals such as lead and mercury were removed by various degrees of imperfection from gold. They further asserted that these base metals could be transmuted to gold by blending them with a substance even more perfect than gold. This elusive substance was referred to as the "philosopher's stone".