152 



f()iuliu'li\"il\' of t'rccziiii;' water, docs not sta\' coiislaiil ; il 

 increases as more and more ice is formed. The formula I), 

 therefore, sliouhl l)e considered correct only in a first a])- 

 })roxiniati()n. The other factoi's tliat we mentioned a])Ove 

 as al'fcclinii,' tlie h'li^th ot" llic cui-n-c should also ])e taken 

 into account in the study of the area limited by the curve. 



Theoretically, at the points of junction of the three curvx's 

 AB, BC and CD, there should ])e sharp angles; practically, 

 the angles ai-e rounded on account, mostly, of the lag in 

 the conduction of heat. As we shall see later, with bio- 

 logical material the j^assage from one section of the curve 

 into another is sometimes so gradual that it cannot be 

 assigned to any definite point. 



We said nothing in this discussion on the heat capacity 

 of the thermometer, of the stirrer and of the containei-. 

 The presence of these objects might sometimes have an 

 important disturbing influence on the shape of the curve 

 and it is not enough simply to correct the results by intro- 

 ducing their water-equivalent in the calculations. 



2. Freezing Curves of Solutions. When freezing bio- 

 logical matei'ial we are always concerned with solutions 

 or suspensions and not with pure liquids. The freezing 

 curve of an aqueous solution in ideal conditions is repre- 

 sented in Figure 18. Such a curve consists of 4 limbs AB, 

 BC, CD, DE. 



The parts AB and DE are simple cooling curves of the 

 exponential type studied in the preceding section. 



The part BC requires a detailed analysis. When the 

 temperature has dropped to a certain point below zero, 

 water starts to freeze out of the solution. This point (B 

 in the figure) is called, though improperly, the freezing- 

 point of the solution. The portion of water which freezes 

 at B liberates heat and the drop in temperature is retarded. 

 However, the curve cannot stay horizontal for any length 

 of time ; as soon as some of the water is solidified, the solu- 

 tion becomes more concentrated and its freezing point is 

 lowered. When the temperature is down to this lower 

 freezing point, moie water freezes, more heat is liberated, 



