x HEAT 147 



experiment oy lowering a piece of cold wire gauze upon an 

 ordinary Bunsen flame. What happens '>. 



Liquids are bad conductors of heat. We can show this in the 

 case of water by the following experiment. 



EXPT. 144. Fill a test-tube three quarters full with cold 

 water, and having weighted a small piece of ice by winding 

 wire round it, or in some other way, drop it into the test-tube. 

 Hold the test-tube near the bottom where the piece of ice 

 is, and warm the top of the water in a Bunsen flame, as shown 

 in Fig. 64. The water at the top can be heated until it boils 

 vigorously and yet the ice Is not 

 melted, showing what a bad 

 conductor the water is. 



Liquids are bad conductors of 

 heat, but gases are far worse, as 

 the following experiments in- 

 dicate : 



EXPT. 145. Examine the 

 shadow of a red-hot poker. 



Notice that the heating of the FJQ 64 ._ To illustrate that Water 

 air as exhibited by its flicker- is a bad Conductor of Heat. 



ing extends but a very little way 

 downwards, thus showing that air is a bad conductor of heat. 



EXPT. 146. Place a little lime in the palm of the hand and 

 bring the point of the hot poker upon it. The air enclosed in 

 the lime does not conduct the heat of the poker, so the hand 

 is not burnt. 



Everyday applications of these facts about Bad Con- 

 ductors. To keep ice in the warm days of summer the custom 

 is to wrap it up in flannel and put it into a refrigerator. The 

 flannel, because of its loose texture, encloses a quantity of air, 

 which, being a bad conductor of he^at, prevents the passage of 

 heat from the warm outside air to the cold ice inside. 



Similarly, ice which has to be conveyed by rail or boat is 

 packed in sawdust. 



The refrigerator itself, too, depends upon much the same 

 facts. The common form consists of a double-walled box with 

 a space between the walls. This is either left "empty" as it is 

 called, when it is full of air ; or, it is filled with some other bad 

 conductor, such as the mineral substance asbestos. 



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