HEAT NEEDED TO MELT SUBSTANCES 39 



than the mere forecasting of local weather conditions. The 

 close relation between humidity and health has led many 

 institutions, such as hospitals, schools, and factories, to regu- 

 late the humidity of the atmosphere as carefully as they do 

 the temperature. Too great humidity is enervating, and not 

 conducive to either mental or physical exertion ; on the other 

 hand, too dry air is equally harmful. In summer the humid- 

 ity conditions cannot be well regulated, but in winter, when 

 houses are artificially heated, the humidity of a room can be 

 increased by placing pans of water near the registers or on 

 radiators. 



30. Heat Needed to Melt Substances. If a spoon is placed 

 in a vessel of hot water for a few seconds and then removed, 

 it will be warmer than before it was placed in the hot water. 

 If a lump of melting ice is placed in the vessel of hot water 

 and then removed, the ice will not be warmer than before, 

 but there will be less of it. The heat of the water has been 

 used in melting the ice, not in changing its temperature. 



If, on a bitter cold day, a pail of snow is brought into a 

 warm room and a thermometer is placed in the snow, the 

 temperature rises gradually until 32 F. is reached, when it 

 becomes stationary, and the snow begins to melt. If the pail 

 is put on the fire, the temperature still remains 32 F., but the 

 snow melts more rapidly. As soon as all the snow is com- 

 pletely melted, however, the temperature begins to rise and 

 rises steadily until the water boils, when it again becomes sta- 

 tionary and remains so during the passage of water into vapor. 



We see that heat must be supplied to ice at o C. or 32 

 F. in order to change it into water, and further, that the 

 temperature of the mixture does not rise so long as any ice is 

 present, no matter how much heat is supplied. The amount 

 of heat necessary to melt I gram of ice fe easily calculated. 

 (See Laboratory Manual.) 



