IN POPULAR PHYSICS. 53 



20. How low a degree of temperature can be reached 

 with a mercurial thermometer ? 



Nearly to the freezing point of mercury, 39 F. 



21. If the temperature be 7O F., ivhat is it C. ? 



70 -32 = 38. 38-^1.8 = 21.1 C. 

 If the temperature be 7O C., what is it F. ? 

 70 x 1.8 = 126. 126 + 32 = 158 F. 



22. Will dew form on an iron bridge ? 

 Yes, because iron is a good radiator. 



On a planJt walk ? 



Not so readily, because wood is a poorer radiator. 



23. Why will not corn pop when very dry ? 



The pores shrink, and the corn becomes compact ; only por- 

 ous, tender-celled corn will pop. 



24. When the interior of the earth is so hot, why do we 

 get the coldest water from a deep well? 



The well extends below the influence of the sun, and not 

 deep enough to be affected by the internal heat of the earth. 



25. Ought the bottom of a tea-Kettle to be polished ? 



No, since a polished surface would reflect the heat. We 

 need a black, rough, sooty surface to absorb the heat rapidly. 



26. Which boils the sooner, milk or water ? 



Milk, because it is so adhesive that the bubbles of steam 

 which are formed at the bottom of the dish can not easily es- 

 cape. They therefore pile up on top of each other, and the 

 milk boils over readily. 



27. Is it economy to keep our stoves highly polished? 



The stove-blacking used is a good radiator, but the surface 

 should not be highly polished, as that hinders radiation. 



28. If a thermometer be held in a running stream, will 

 it indicate the same temperature that it would in a pailful 

 of the same ivater ? 



