LESSONS IN PHYSICS. 31 



While this process is called absorption, it is practically the same 

 as conduction. 



Con vection is the process by which one body carries heat from 

 its source to another body. Air receives heat from the stove, 

 moves to other parts of the room and gives up the heat to other 

 bodies. Liquids and gases are good conveyers of heat. 



Experiment: Take a test tube and fill it nearly full of dirty 

 water, and, inclining it slightly, apply heat to the bottom and 

 very soon currents will be observed in the tube, and if we test 

 with the finger the temperature of this water, we shall find that 

 there is a warm current on the upper side of the tube and a 

 cooler current on the lower side; now remove the heat and hold 

 the tube in a vertical position and it will be found that the 

 water at the top is warmer than the w r ater at the bottom of the 

 tube. The water has conveyed the heat from the bottom of the 

 tube to the top by the process of convection. 



Heat from the sun will pass through the air and window glass, 

 and warm the window sill or the hand much more than it warms 

 the glass or the air. The property by which some bodies allow 

 heat to pass through them without themselves being much 

 warmed, is called diathermancy. Such bodies are called diather- 

 mic bodies. They are transparent to heat as some bodies are 

 transparent to light. 



