386 Of the Propagation of Heat 



The result will be the same, when, to prevent the 

 communication of Heat from the air during the experi- 

 ment, that part of the bulb of the thermometer (the su- 

 perior half of it) which projects above the level of the 

 top of the tumbler is covered with a sheath lined with 

 soft fur. 



When more or less than half of the bulb of the ther- 

 mometer is plunged into the ice and water, the height 

 of the liquid in the tube of the instrument will show 

 that that part only of the Fluid in the bulb is cooled 

 which occupies the part of the bulb that is immersed in 

 the ice and water. 



§ 2. Ice-cold Water^ standing on Ice, may be heated and made 

 to boil without melting the Ice, contrary to an Opinion 

 that has generally prevailed. 



Take a thin glass tube i inch in diameter, and about 

 8 or 10 inches long, containing about two or three 

 inches of water, and by plunging the end of the tube 

 into a freezing mixture of pounded ice and sea-salt 

 cause the water in the tube to congeal ; this being done, 

 pour two or three inches of ice-cold water on the ice ; 

 and wrapping up about two inches of the lower end of 

 the tube with a piece of flannel, and holding it inclined 

 at an angle of about 45°, by that part of it which is so 

 covered, bring that part of the tube which is at the 

 height of the surface of the fluid-water to be just over 

 the point of the flame of a burning candle, and distant 

 from it about two or three inches. When the water in 

 that part of the tube begins to boil, the tube may be ad- 

 vanced slowly over the flame of the candle ; and if due 

 care be taken to prevent a too sudden application of 



