254 METALS AND THEIR COMBINATIONS. 



While alloys are generally looked upon as molecular mixtures, and 

 not as definite chemical compounds, yet there are many alloys the 

 properties of which are not intermediate between those of the elements 

 entering into these alloys, as we should expect if they were mechanical 

 mixtures. For this reason it is assumed that, in at least some cases, 

 compounds are formed which, however, are generally dissolved in, or 

 mixed with, an excess of one of the constituent metals. 



On the other hand, there are cases where there is an utter lack of 

 affinity between the component parts of an alloy. Thus, alloys of 

 copper and lead, usually termed pot-metal alloys, show particles of the 

 two metals side by side, when the fractured surface is examined with 

 the microscope. 



Manufacture of alloys. Alloys are generally obtained by fusing the 

 metals together ; but in order to do it successfully such properties of the com- 

 ponents as fusibility, specific gravity, proneness to oxidize, etc., should be con- 

 sidered. As a general rule the metal having the highest fusing-point is melted 

 first, and to it are added the other metals in the diminishing order of their 

 fusing- points. Loss or deterioration by oxidation should be guarded against 

 by covering the surface of the liquid mass with charcoal or with such fluxes as 

 borax, sodium chloride, or ammonium chloride. The heat should at no time 

 be higher than is necessary for the liquefaction. 



Properties of alloys. Alloys generally are harder and more brittle, but 

 less ductile and malleable than the constituent metals possessing these qualities 

 in the highest degree. The union even of two ductile metals may destroy that 

 property more or less completely, as is shown by the absence of ductility in an 

 alloy of gold and a small portion of lead. The combination of a brittle and a 

 ductile metal always yields a brittle alloy. 



Tenacity is generally increased. Thus, copper alloyed with 12 per cent, of 

 tin has its tenacity trippled ; gold, when alloyed with copper, silver, or plat- 

 inum, has its tensile resistance nearly doubled ; aluminum bronze, an alloy of 

 copper and aluminum, has a greater tenacity than that of either of the con- 

 stituent metals. 



Certain metals impart to alloys specific properties. Thus, bismuth and 

 cadmium increase fusibility; tin and lead, both of which are soft metals, 

 impart hardness and tenacity ; arsenic and antimony produce brittle alloys. 



QUESTIONS. How many metals are known, and about how many are of gen- 

 eral interest? Mention some metals having very low and some having very 

 high fusing-points. What range of specific gravities do we find among the 

 metals ? Mention some univalent and some bivalent metals ; also some which 

 show a different valence under different conditions. Mention some metals which 

 are found in nature in an uncombined state; some which are found as oxides, 

 sulphides, chlorides, and carbonates, respectively. Into what two groups are 

 the metals divided? State the three groups of light metals. What is a metal ? 

 What is an alloy, and what is an amalgam ? By what process can most metals 

 be obtained from their oxides? 



