MODES OF DIFFUSING HEAT. 



345 



(a.) Cork the neck of a glass funnel and pass the tube of an 

 inverted thermometer through the cork, or use an air 

 thermometer, as shown in the figure. Cover the ther- 

 mometer bulb to the depth of about half an inch with 

 water. Upon the water pour a little sulphuric ether 

 and ignite it. The heat of the flame will be intense 

 enough to boil a small quantity of water held over it, 

 but the thermometer below will be scarcely affected. 

 Fasten a piece of ice at the bottom of a glass tube, 

 and cover it to the depth of several inches with water. 

 Hold the tube at an angle of about 45, and apply the 

 flame of a lamp below the upper part of the water. 

 The -water there may be made to boil without melting 

 the ice. The conductivity of gases is probably lower 

 FIG. 259. than that of liquids. 



541. Convection. Fluids (with the exception oi 

 mercury, which is a metal) being poor conductors, they 

 cannot be heated as solids gen- 

 erally are. Water, e.g., must be 

 heated from below; the heated 

 molecules expand and rise while 

 the cooler ones descend to take 

 their place at the source of heat. 

 These currents in heating water 

 may be made visible by dropping 

 a small quantity of cochineal or 

 oak sawdust into the vessel con- 

 taining the water. TJiis method 

 of diffusing heat, "by actual 

 motion of heated fluid masses, 

 is called convection. Expansion 

 by heat and the force of gravity are essential to convection. 

 Since aeriform bodies are expanded more by heat than 

 liquids are, these currents of heated gases are more active 

 than those of liquids. Hence the drafts of lamps and 

 stoves, the existence of trade winds, etc. 



FIG. 260. 



