/2V NATUKAL PHILOSOPHY. 7 



The pressure of the liquid in the bottle is less, and therefore* 

 they form more slowly. 



6. Why are drops larger if you drop them slowly ? 



There is more time for the adhesive force of the bottle to 

 act on the liquid, and so a larger drop can be gathered. 



7. Why is a tube stronger than a rod of the same weight? 

 Let a rod supported at both ends be broken in the middle. 



We shall see that it yields first on the circumference. So true 

 is this, that long beams heavily loaded have been broken by a 

 mere scratch of a pin on the lower side. The particles along 

 the centre break last. They rather aid in the fracture, since 

 they afford a fulcrum for the rest of the rod, acting as the long 

 arms of a lever, to act upon. In a tube the particles at the 

 centre are removed and all concentrated at the outside, where 

 the first strain is felt. (See Physiology, p. 20). 



8. Why, if you melt scraps of zinc ', will they form a solid 

 mass when cooled? 



The heat overcomes, in part, the attraction of cohesion, so 

 that the particles flow freely on each other. They now all 

 come within the range of cohesion, so that when the metal 

 cools they are held by that force in a solid mass. 



9. In what liquids is the force of cohesion greatest? 

 Mercury, molasses, etc. 



10. Name some solids that will volatilize without melting f 

 Arsenic, camphor. 



ADHESION. 



47. i. Why does cloth shrink when wet? 



By capillary attraction the water is drawn into the pores 01 

 the cloth. The fibres are thus expanded sidewise and short- 

 ened lengthwise. The cloth "fulls up" or thickens while it 

 shortens and narrows (shrinks) in the process. 



2. Why do sailors at a boat-race wet the sails ? 

 The pores being full and expanded make the sails more com- 

 pact. They will therefore hold the wind better. 



