Metals. The division of the elements into the two categories, the metals and the non-metals, is one which, though not scientific or rigid, is yet so convenient as to make it commonly adopted. Of the metals now known, six were familiar to the ancients and are mentioned in the scriptural writings. These are gold, silver, tin, copper, lead, and iron. Other metals became gradually added to the list, and at the commencement of the present century the number of metals known to chemists was raised to seventeen; while now, at its close, this number has been more than trebled. The first definition of the term was given by Geber in the 8th century, who regarded fusibility and malleability as the essential characteristics. According to his view also of the nature of these substances, they all consisted of varying quantities of mercury and sulphur, and he and the following alchemists were therefore great believers in the possibility of transmuting the common metals into gold. When the brittle metals, antimony, etc., were discovered, they were regarded as only semimetallic or pseudometallic, being not malleable, and mercury was not regarded as a metal until it had been solidified. In the 18th century the distinction between the metals and semimetals was lost, and the characteristic properties of metals were opacity, lustre, and high specific gravity. The last of these qualities had to be given up as non-essential when sodium and other light metals were discovered; while the first two have also disappeared, as many non-metals possess a metallic lustre and all are transparent if sufficiently thin. The 18th-century idea of the chemical nature was that they all consisted of a metallic calx united with phlogiston [Phlogiston], and it was left to Lavoisier to demonstrate their elementary nature. At present a complete definition to include all metals is difficult. They are best characterised as being elements the oxides or at least the lower oxides of which form bases capable of neutralising acids with the formation of salts.