METALS AND SOME OF THEIR COMPOUNDS 259 



with the names of the seven " planets." Of these names Sol 

 (gold), Luna (silver), Mercury (quicksilver), Venus (copper), 

 Mars (iron), Jupiter (tin), Saturn (lead), only one, namely Mer- 

 cury, has been retained in common use. The crude practices in 

 the laboratories of the alchemists led to a few useful discoveries, 

 among them probably the metal zinc, which was first mentioned 

 by Paracelsus in the sixteenth century. In the latter half of the 

 eighteenth century, at the time of Lavoisier, the number of 

 recognised metals was seventeen. The popular idea at that time 

 was, and is down to the present day, that a metal is a hard, 

 shining, and heavy substance, which can be melted only in a 

 hot fire. When therefore Davy in 1808 discovered potassium 

 and sodium, which are both lighter than water, some perplexity 

 was caused by their anomalous qualities and for a time it was 

 proposed to designate them merely metalloids. This term, 

 however, with the authority of Berzelius, soon received a dif- 

 ferent application, and with further knowledge of the physical 

 and chemical properties of these elements they were included in 

 the category of metals. Somewhere about fifty substances are 

 now called metals, but it would be difficult to secure complete 

 unanimity among chemists as to whether particular elements 

 should be included. There is, however, one test which would 

 probably be accepted generally. In the process of electrolysis 

 the metals are always deposited at the cathode and are therefore 

 spoken of as positive elements, notwithstanding differences in 

 other physical characters, such as density, fusibility, ductility, or 

 brittleness. 



Gold, which occurs in nature almost always in the native or 

 metallic state, was probably the first known to primeval man. 

 The others occur chiefly in the form of sulphides or oxides, and 

 the common useful metals are for the most part obtained by 

 reduction of their oxides. Even those which, like copper, lead, 

 and zinc, are found in combination with sulphur are usually 

 submitted to a preliminary process of roasting in contact with 

 air, so that much of the sulphur is burnt off and an oxide of 

 the metal remains which is subjected to further treatment. 



The art of metallurgy has, however, undergone great develop- 

 ments and many modifications within recent years, owing 

 especially to the introduction of the electric arc, which gives 

 temperatures far above ordinary furnace heat, and the electric 

 current- by which the method of electrolysis can now be applied 



