146 GENERAL SCIENCE 



pendently of air currents or any conducting matter is 

 called radiation. 



It will be well for us to note some of the differences 

 between conduction, convection, and radiation. 



Conduction and convection are comparatively slow, 

 while radiation is rapid. The sun's heat comes to us 

 with the enormous speed of light, 186,000 miles a second. 

 The heat which comes to us by radiation comes in straight 

 lines, while conducted or convected heat may come by the 

 most roundabout paths. A screen placed between the 

 source of radiant heat and a body will cut off the heat from 

 the body. 



Radiant heat will pass through certain media without 

 heating them. The heat from the sun will pass through 

 ordinary window glass and leave it much colder than the 

 objects it falls upon inside the room. Also the upper 

 regions of the atmosphere are very cold even in the hottest 

 time of the year. 



QUESTIONS AND PROBLEMS 



1. Tubs of water are sometimes put in cellars in cold weather 

 that the freezing of the water may prevent the freezing of the vege- 

 tables. Explain this phenomenon. 



2. Why is a quart of heated water a better foot-warmer than an 

 equal volume of heated lead ? 



3. How did " latent heat " get its name? 



4. 100 grams of water at 10 C. are mixed with 200 grams of 

 water at 50 C. What is the resulting temperature? 



5. If 20 grams of ice at are mixed with 100 grams of water at 

 80 what will be the temperature? 



6. Why do we put salt on ice in an ice-cream freezer? 



7. What is the temperature of a mixture of ice and water? 



8. What must be the temperature of the water if when equal 

 weights of water and ice at are mixed the result is water at ? 



9. Water boils at 90 C. at Quito. How do you account for 

 the low boiling point? 



