The Propagation of Heat 



187 



substances placed in the interior of the flask lose their 

 heat very slowly, since conduction and convection 

 (described below) are almost entirely prevented and 

 the silvering of the glass further checks the loss which 

 might occur by other means. 



124. The general mode of propagation of heat in 

 fluids is illustrated in the following experiments. 



EXPERIMENT 68. Nearly fill a large beaker with 

 water and place it on a piece of wire gauze on a tripod 

 stand. Tear some blotting-paper into very small pieces 

 and drop them into the water, allowing them to become 

 completely soaked and fall to the bottom of the beaker. 

 Heat the water by placing a small Bunsen flame under 

 the edge of the beaker. The heated water will move 

 upwards from the point of application of the heat and 

 cooler water will take its place, the direction of motion 

 of the water being indicated by that of 

 the scraps of blotting-paper, provided 

 that these are small enough. 



Heat is here transmitted from one part 

 of the liquid to another by actual motion 

 of the heated portions of the water away 

 from the source of heat, their place being 

 taken by cooler water which becomes 

 heated and moves away in turn. By 

 continued repetition of this process the 

 temperature is caused to rise in all parts 

 of the liquid. Buildings are frequently 

 heated by taking advantage of this 

 motion of the hot portion of a liquid 

 to a cooler region and the method em- 

 ployed is roughly outlined in the next experiment. 



EXPERIMENT 69. Fit up the apparatus of which 

 Fig. 58 is an illustration. The upper vessel is a 



Fig. 58. 



