166 Domestic Science 



to 20, 58 x 20= 1160 calories must be supplied. 

 Since it took about four times as long to melt the ice, 

 using the same source of heat, the quantity of heat 

 supplied during the melting must have been about 

 4 x 1160 = 4640 calories. Hence to melt 1 g. of ,ice, 

 without producing any rise of temperature, 4640 -^ 58 

 = 80 calories have been used. This quantity of heat, 

 used in merely changing the physical state of the ice, 

 is said to be " latent " in the water formed, and the 

 " latent heat of fusion of ice " is said to be- 80. The 

 word "latent" means "hidden", and, if we performed 

 the above experiment in the reverse order, the suit- 

 ability of the name would be evident. It is not easy 

 to obtain measurements with any degree of accuracy 

 in doing this, but a rough idea may be obtained by 

 performing 



EXPERIMENT 55. Pound up some ice in a mortar. 

 Place a layer of this crushed ice at the bottom of a 

 large beaker, and cover it with a layer of ordinary 

 salt, using roughly about half as much salt as ice. Put 

 into the beaker two more layers of each substance. 

 Place a smaller beaker upon the top of the last layer, 

 and surround this beaker with alternate layers of the 

 ice and salt till the level of the mixture reaches the 

 lip of the small beaker. Press the mixture down 

 firmly. Remove the small beaker, wipe it dry, and 

 half fill it with water. Warm the water, if necessary, 

 to 20, and then replace the beaker. Keeping it 

 constantly stirred with a thermometer, note the time 

 taken to cause the temperature of the water to fall 

 to 0. The mixture of ice and salt is much colder 

 than would be the case if ice alone were used, and, 

 if the thermometer be allowed to remain in the water, 

 ice will be formed, but no further fall of temperature 



