When two elements combine and form more than one compound, the masses of one element that react with a fixed mass of the other are in the ratio of small whole numbers.

This was mainly due to a member of the Society of Friends named Robert Dunkin. Joseph Banks had long had his eye on Davy and in February 1801 Davy was officially interviewed by Banks Benjamin Thompson (who had been appointed Count Rumford) and Henry Cavendish the Committee of the Royal Institution. "It [science] has bestowed on him powers which may almost be called creative; which have enabled him to modify and change the beings surrounding him and by his experiments to interrogate nature with power not simply as a scholar passive and seeking only to understand her operations but rather as a master active with his own instruments.

In 1815 he invented the Davy lamp which allowed miners to work safely in the presence of flammable gases. He is probably best remembered today for his discoveries of several alkali and alkaline earth metals as well as contributions to the discoveries of the elemental nature of chlorine and iodine.