238 HEAT. 



the thermometer. The upper surface of a vessel of wa- 

 ter has been made to boil a long time, with a piece of 

 unmelted ice at the bottom. Liquids are found, howev- 

 er, to possess a conducting power in a very slight degree. 

 When a vessel of water is heated in the ordinary- 

 way over a fire, the heat is carried through it merely 

 by the motion of its particles. The lower portion be- 



^. „„„ comes warm and expands ; it immediate- 

 Fig. 200. _ ^ ' 



ly rises to the surface, and colder portions 

 sink down and take its place, to ascend in 

 their turn. In this way, a constant cir- 

 culation is kept up among the particles. 

 These rising and descending currents are 

 shown by the arrows in Fig. 200. This 

 result may be easily shown by filling a flask 

 with water into which a quantity of saw- 

 dust from some green hard wood has been 

 thrown, which is about as heavy as water. 

 It will traverse the vessel in a manner precisely Hke 

 that shown in the figure. 



These results show the importance of applying heat 

 directly to the bottom of all vessels in which water is 

 intended to be heated. A considerable loss of heat oft- 

 en occurs when the flame is made to strike against the 

 sides only of badly-arranged boilers. 



SECTION II. 

 EXPANSION BY HEAT. 



An important effect of heat is the expansion of bod- 

 ies. Among many ways to show it, an iron rod may 

 be so fitted that it will just enter a hole made for the 



