HEAT OF FUSION AND DISSOLUTION 93 



as the heat given out when that substance freezes. So the 

 heat of fusion is the same as the heat of solidification. The 

 heat of fusion of ice is 80 calories. Every time one gram 

 of water at o C. freezes, it gives out 80 calories of heat 

 without changing its temperature. This explains why 

 water freezes so slowly and why ice melts slowly. After 

 water is frozen, the tem- 

 perature of the ice will 

 decrease if the air around 

 it is below oC. The spe- 

 cific heat of ice is .5, that 

 is, when the temperature 

 of one gram of ice is low- 

 ered i C., it gives out 

 one-half cal rie of heat. 

 Glass, unlike some 

 other substances, does 

 not change suddenly into 

 a liquid. After it begins 

 to melt it continues to 



increase in' temperature 



. 111 i BLOWING GLASS 



if heat is added. The 



hotter it is made the easier it will flow. This is important 

 in the manufacture of window panes, as it permits rolling 

 the glass into thin sheets when in a semi-liquid condition. 

 67. When water freezes it expands about one-ninth. 

 Nine gallons of water when frozen will become ten gallons 

 of ice. The weight of nine gallons of water is the same 

 as that of ten gallons of ice. Hence nine gallons of ice 

 weigh less than nine gallons of water. Why does ice float 

 on water? If a block of pure ice is placed in water what 

 part of it will be above the water? Prove your answer 

 by placing some ice in water. 



