THE DISCOVERY OF THE ELEMENTS. 475 



Of the many definite substances known to chemists before tlie dis- 

 covery of hydrogen gas, the following were afterward recognized by 

 Lavoisier and his colleagues as elementary : First, the seven metals 

 known to the ancients, namely, gold, silver, mercury, copper, iron, tin, 

 and lead, distinguished respectively by the signs of the sun, moon, and 

 planets; and each- conceived to have some mystic connection with the 

 particular orb or planet of which it bore the sign, and not unfrequently 

 the name. Then three metals which became known at the latter end 

 of the fifteenth or beginning of the sixteenth century, namely, anti- 

 mony, discovered by Basil Valentine in 1490; bismuth, mentioned by 

 Agricola, 1530; and zinc, mentioned by Paracelsus, obiit 1541. An 

 elementary character was also assigned to the non-metals carbon and 

 sulphur, which had been known from the earliest times ; to phospho- 

 rus, discovered by Brandt, of Hamburg, in 1669 ; and to boracic acid, 

 now known to be a hydrated oxide of boron, first discovered by Hom- 

 berg in 1702, and still occasionally spoken of as Homberg's sedative 

 salt. The list was further swollen by four metals which, in Lavoisier's 

 time, had been but recently discovered, namely, cobalt and arsenic, 

 identified simultaneously in 1733 by George Brandt, of Stockholm; 

 platinum, discovered in 1741 by Woods, assay-master at Jamaica; and 

 nickel, discovered in 1751 by Cronstedt. 



The only other bodies known before 1766, and afterward included 

 in the class of elements, namely, the alkalies and earths, had during 

 the quarter of a century immediately preceding been made the sub- 

 jects of especial study. The differentiation of potash from soda, both 

 previously known by the common name of alkali, was indicated by 

 Duhamel in 1736, and more completely established by Marggraf in 

 1758. The differentiation from one another of lime or calcareous 

 earth, silex or vitrefiable earth, alumina or argillaceous earth, and 

 magnesia or bitter earth, was accomplished by the labor of many 

 chemists, more particularly Marggraf, Bergmann, and Scheele ; prior 

 to whose researches, silex, alumina, and magnesia, together with their 

 different combinations and commixtures with each other and with 

 lime, were held to be but impure varieties of lime. The nature of the 

 difference between the caustic alkalies and earths and their respective 

 carbonates was made known by Black in 1756; while the real consti- 

 tution of the alkalies and earths, as metallic oxides, though suspected 

 by Lavoisier, was not established until the beginning of the present 

 century, by Davy and his contemporaries and followers. 



The successive recognition of the elementary gases, quickly follow- 

 ing Black's remarkable discovery of carbonic-acid gas, began with the 

 identification of hydrogen by Cavendish in 1766. This was succeeded 

 by the discovery of nitrogen by Rutherford in 1772 ; of chlorine and 

 fluoric acid, the latter now held to be a fluoride of hydrogen, by 

 Scheele in 1774 ; and of oxygen by Priestley in the same year. 



Thus prior to the discovery of the first of the elementary gases, 23 



