154 



SCIENCE OF COMMON THINGS. 



How liquids are made hot. 



Why water is agitated when boiling. 



Fig. 38. 



surface, and the other of colder particles 

 descending to the bottom. The portion of 

 liquid which receives heat from below is 

 thus continually mixed through the other 

 parts, and the heat is diffused by the mo- 

 tion of the particles among each other. 



These currents take place so rapidly, that if a thermo- 

 meter be placed at the bottom and another at the top 

 of a long jar, the fire being applied below, the upper one 

 will begin to rise almost as soon as the lower one. The 

 movement of the particles of water in boiling will be 

 understood by reference to Fig, 38. 



995 What common experiment proves that water is a bad conductor of 

 heat f 



When a blacksmith immerses his red-hot iron in a 

 tank of water, the water which surrounds the iron is 

 made boiling hot, while the water not immediately in 

 contact with it remains quite cold. 



If a tube nearly filled with water is 

 held over a spirit lamp, as in Fig. 39, in 

 such a manner as to direct the flame 

 against the upper layers of the water, 

 the water will be observed to boil at the 

 top, but remain cool below. If quicksil- 

 ver, on the contrary, be so treated, its 

 lower layers will speedily become heated. 

 The particles of mercury will communi- 

 cate the heat to each other, but the particles of water will not do so. 



996 "Why is water in such continual ferment when it is boiling ? 



This commotion is mainly produced by the ascend- 

 ing and descending currents of hot and cold water. 



The escape of steam from the water contributes also to increase this 

 agitation. 



997* How do these two currents pass each oilier f 



The hot ascending current rises up through the centre 

 of the mass of water ; while the cold descending cur- 

 rents pass down by the metal sides of the kettle. 



998 Why is heat applied to the bottom, and not to the top of the kettle ? 



Because the heated water always ascends to the sur- 

 face^ heating the water through which it passes ; if, 

 therefore, heat were applied to the top of a vessel, the 

 water below the surface would be heated very slowly. 



Fig. 39. 



