IN POPULAR PHYSICS. 11 



4. Why do drops of different liquids vary in size ? 



Because they vary in cohesive force. 



5. Why, when you drop medicine, will the last few 

 drops contained in the bottle be of a larger size than the 

 others ? 



The pressure of the liquid in the bottle is less, and there- 

 fore they form more slowly. 



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



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, if you melt scraps of zinc, will they form a 

 solid mass w/ien 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. 



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



Mercury, molasses, etc. 



9. Name some solids that will volatilize tvithout melt- 

 iny. 



Arsenic, camphor. 



10. Why can glass be ivelded ? 



Because, like iron, it becomes viscous before melting. 



11. Name some substances that can not be ivelded. Why 

 not? 



Wood can not be welded, nor can lead or bismuth. They 

 can not be made to assume the viscous condition. 



12. What liquids would you select for showing surface 

 tension ? 



Solution of soap in water is the most convenient. The dif- 

 ference in tension between films of different kinds of liquid is 

 well shown by carefully dropping oil of coriander or oil of cin- 

 namon, or minute fragments of clean camphor on the surface 

 of perfectly clean water. 



