IN PHYSICS. 207 



20. Show that the law of the diffusion of gases aids in 

 preserving the purity of the atmosphere. 



(See New Chemistry, p. 96.) 



Foul gases do not remain for any length of time in one 

 place, but tend to spread through the adjacent atmos- 

 phere. Fresh air also seeks to creep into noisome 

 localities. 



21. In casting large cannon, the gun is cooled by a stream 

 of cold water. Why ? 



The object of this is to cause the iron to cool more 

 quickly and so not give the molecules time to arrange 

 themselves in crystals. 



(See p. 45.) 



22. Why does paint adhere to wood? Chalk to the 

 blackboard ? 



These are illustrations of the force of adhesion. 



23. Why does a towel dry one's face after washing 1 } 



The capillary pores of the cloth absorb the water on 

 the face. 



24. Why will a greased needle float on water ? 



The repulsion between the grease and the water is 

 sufficient to support the slight weight of the needle. 



2 5 . Why is the point of a pen slit ? 



The ink rises in the capillary space of the slit, and is 

 there held for use. When the pen is pressed on the 

 paper, the space is widened and the ink descends. 



26. Why is a thin layer of glue stronger than a thick 

 one? 



