

96 ANSWERS TO PRACTICAL QUESTIONS 



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



See Question 20. 



22. Show that the earth's crust is mainly composed of 



burnt metals. 



(See Cooke's Religion and Chemistry.) 



It consists largely of potassium, magnesium, calcium, alu- 

 minum, sodium, etc., in combination with O. These compounds 

 are the products of combustion. 



The elements O, Si, Al, Mg, Ca, K, Na, Fe, C, S, H, Cl, 

 and N 13 in all probably make up T 9 ^ of the earth's crust. 



23. What kind of iron is used for a magnet ? For a 

 magnetic needle ? 



Steel for a permanent magnet, and therefore for a magnetic 

 needle ; pure soft iron for an electro-magnet. 



24. Why does a "tin" pail so quickly rust out when 

 once the tin is worn through ? 



These pails are made of sheet iron, which is covered with a 

 coating of tin, which causes the popular name for them. If 

 this is scratched through, the iron and tin in contact are ex- 

 posed to the water ; voltaic action is started, and the iron rap- 

 idly rusts. 



25. Wlty is the zinc oacide found in Neiv Jersey red 9 

 when zinc rust is white ? 



The oxide in New Jersey is colored by compounds of iron 

 and manganese. 



26. Should we filter a solution of permanganate of 

 potash through paper ? 



(See Chemistry, p. 157, note.) 

 No. The salt will give up O and corrode the filter. 



27 Why is woody cordage, etc., sometimes soaked in a 

 solution of corrosive sublimate ? 



This salt possesses strong antiseptic properties. 



28. Why does the ivhite paint around a sink turn 

 black? 



