149 



B' 



Fig. 17. Freezing curve of a liquid under ideal conditions. Abscissae: 

 time ; ordinates : temperature. 



represents the cooling of water, and the curve CD the 

 cooling of ice, that is, of a solid which cannot be stirred, 

 and which possesses a heat conductivity about 4 times 

 higher and a heat capacity about twice lower than that of 

 water. The impossibility of stirring results in a non- 

 uniform distribution of temperature ; in a solid body which 

 is being cooled the temperature decreases gradually from 

 the center to the periphery. The high heat conductivity 

 and the low specific heat of ice cause a more rapid cooling 

 than in the case of water; consequently, the slope of the 

 curve CD is steeper than that of AB. 



To construct exponential curves one needs only two ex- 

 perimental points. Let us assume, for example, that a 

 mass of water at + 10° is exposed to a bath at - 5°, and that 

 the cooling rate is such that the temperature drops during 

 the first minute from 10° to 8°, that is, if - 5° is taken as the 

 origin, from 15° to 13° (from 10 + 5° to 8 + 5°). The two 

 experimental values, 13 and 15, give the ratio of the geo- 



