IN CHEMISTRY. 12 5 



14. Why does silver blacken if handled? 



The perspiration of the body contains S, which combin- 

 ing with the metal forms silver sulphide the black sul- 

 phuret of silver. 



15. Why does silver tarnish rapidly where coal is used 

 for fires ? 



S, which is present in coal, is set free by combustion 

 and forms a silver sulphide. 



1 6. Why is a solution of a coin blue? 



From the Cu which is contained in silver coin. 



17. Why will a solution of silver nitrate curdle brine ? 

 A white, curdy precipitate of silver chloride is formed. 



1 8. Why does writing with indelible ink turn black when 

 exposed to the sun, or to a hot iron ? 



By the decomposition of the silver salt contained in the 

 ink, and consequent production of Ag s O, which stains 

 organic matter black. 



1 9. What alloys resemble gold ? 

 Oreide, aluminum-bronze, etc. 



20. Why does a fish-hook " rust out " the line to which it 

 is fastened ? 



Ferric oxide and ferric hydrate act as conveyers of O, 

 absorbing it from the air and giving it up to organic 

 bodies with which they are in contact. 



21. Why do the nails in clap-boards loosen ? 



(See Question 20.) 



