no Malleability of Mel ah. [Book VI. 



are perfectly opake. The opacity of metallic bodies 

 renders them exceedingly proper for reflecting the 

 rays of light, as no bodies poffefs this property in fo 

 eminent a degree. Thus we obferve that ghfs reflects 

 objects very imperfectly, imlefs coated with metal, as 

 in looking-glalTes. The capacity of reflecting light 

 depends on the degree of polifh and the whitenefs of 

 furface, confequently white metallic fubftances reflect a 

 greater quantity of light, and are more brilliant, than 

 thofe which are coloured. 



A property which feems to belong exclufively to 

 metals, and yet not to all of them, is malleability. This 

 confifts in a capacity of having their fubftance ex- 

 tended, and their furface increafed, either in breadth 

 or length, without being liable to fracture. The 

 capacity, however, of being extended in length and 

 breadth is not exactly the fame, for metals which 

 admit of extenfion under the hammer cannot always 

 be drawn into wire, which property is diftinguifhed 

 from the other by the term ductility. Beating always 

 reduces metals to a (late of rigidity and brittlenefs. 

 To remedy this, the metals are generally heated red 

 hot, and cooled again flowly j in this cafe their mal- 

 leability will be increafed, and this is called annealing 

 them. Crammer fays, that if gold is annealed in this 

 manner the leaf will be opake, and that the tranfmif- 

 lion of light through ordinary gold leaf depends on 

 fmall cracks and flaws in the metal. If this afiertion 

 is true, it will in a great meafure eftablifli the perfect 

 opacity of metallic fubftances. Metals, when ham- 

 mered, give out a degree of heat, and at", the fame 

 time become brittle ; and the only methoc] of reftoring 

 their malleability is to expofe them to the action of 

 heat, and to fiiffer as much of this as poffible to fix 

 idelfin their fubftance, by cooling them gradually. 



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