54 



CHEMISTRY. 



and lead ; solder, from tin and lead ; gold and silver coins are also 

 alloys. Many of the alloys occur native. 



But few of the metals are found native, that is, in their uncom- 

 bined form. They usually are united with either oxygen or sulphur, 

 or else they occur as salts. 



Metals differ greatly in their attraction for oxygen ; some have 

 such a powerful affinity for this agent, that they decompose water, 

 in order to unite with it. On the other hand, gold, silver, and pla- 

 tinum are very difficult to oxidize. The term noble has been given 

 to such metals as are not liable to tarnish on exposure to the air. 



Most metals may be oxidized by exposure to heat in the open air. 

 This process was formerly termed calcinatio7i; and the product 

 formed, a calx. Another mode of oxidizing is by deflagration, or 

 by heating with nitrate of potassa, or chlorate of potassa. Nitric 

 and nitro-hydrochloric acids are also powerful oxidizing agents. 



Metallic oxides may be reduced either by heat alone, as in the 

 case of the oxides of the noble metals, or by the united agency of 

 heat and combustible matter, as hydrogen, charcoal, and the black 

 flux; or by galvanism, which is still more powerful; or by the 

 agency of deoxidizing agents, as phosphorous acid, protochloride of 

 tin, &c. ; or by precipitation from their solutions by means of other 

 metals ; thus silver is thrown down from a solution of its nitrate by 

 means of mercury ; copper, by means of iron, and so on. 



The compounds of metals with oxj^gen, are for the most part 

 oxides; occasionally, they are acids. Arsenic is the only one which 

 forms an acid, without, at the same time, forming an oxide. Most 

 of the metallic oxides act as salifiable bases — that is, form salts with 

 acids. Generally, but not always, this property is confined to the 

 protoxides. 



Chlorine, bromine, iodine, and fluorine have also a strong affinity 

 for metals, — particularly the two former. 



Sulphur has a strong tendency to unite with metals : the combi- 

 nation may be effected either by heating the powdered metal and 

 sulphur together; by igniting together a metallic oxide and sulphur; 

 by heating a sulphate, along with combustible matter, by means of 

 which the oxygen is removed in the form of carbonic acid, or by 

 the action of hydrosulphuric acid. Several of the metallic sulphurets 

 (sulphides) occur native, as of lead, antimony, iron, zinc, &c. 



The compounds of selenium and the metals closely resemble the 

 sulphurets. 



Phosphorus and hydrogen occasionally unite with the metals. 



CLASSIFICATION OF THE METALS. 

 Class I. Metals of the Alkalies. 

 " \\. Metah of the Alkaline Earths. 

 " III. Metals of the Earths. 

 " IV. Metals proper. 



