LECTURE XXXII. 



CHEMISTRY. 



METALS. 



METALLIC substances are valuable for their 

 durability in resisting the effects of humidity and 

 of air : for the closeness of their texture, which 

 renders them capable of polish ; for the strong 

 cohesion of their particles, which renders them 

 highly proper for utensils where strength is to be 

 combined with moderate bulk. 



The first and most obvious property of metals 

 is their remarkable weight in general ; but whilst 

 in some their specific gravity is very great, it 

 being in platinum 21,5, in potassium it is so low 

 as 0,865. 



2. They have a great degree of opacity, throw- 

 ing off all the rays of light without receiving any 

 into their substances; and the thinnest film of 

 metal is as perfectly opake as any solid mass. It 

 has been thought that gold leaf, which transmits 

 some rays of light, is an exception ; but there is 

 reason to think that this depends not on any 

 pores in the metal, but on cracks and flaws pro- 

 duced by beating, which always reduces metals, 

 to a degree of rigidity and brittleness. To re- 



