248 fee and its Natural History 



the straight line Ba represents the variation of V with /, the 

 salt being inert. The line B# represents the dilatation of 

 pure ice, on the basis of a constant coefficient of dilatation 

 O'oooid As chloride of sodium is, by nature, inert at tem- 

 peratures below the cryohydric point, the straight line be is 

 part of the curve B, as well as of Ea. Between 25 and 



2i72 the ice expands uniformly at the same rate, whether 

 it is pure or contains salt. We have in c the first singular 

 point of the curve, and it occurs in all the curves of expansion 

 of ice contaminated by chloride of sodium. When the tem- 

 perature of the ice rises to 2i72, the inertness of the salt 

 is exchanged for activity; and, if the requisite supply of heat 

 is available, 5*498 c.c. of ice are melted, producing 5^040 c.c. 

 water, under a contraction of 0*458 c.c. While this amount 

 of ice is melting, the temperature remains constant, but the 

 volume U contracts. Graphically, this is represented by a 

 straight line (ce) parallel to the axis of volume. Therefore 

 the curve of volume of the ice between 25 and the point 

 where the temperature begins to rise above -21 72 is repre- 

 sented by two straight lines be and ce which meet each other 

 in an acute angle at c. When the temperature rises above 



21 72, another, generally acute, angle is formed at e, so 

 that this portion of the curve of volumes takes the form of 

 the letter Z. 



Between 25 and -2i72i the coefficient of dilatation 

 is O'OOOi6. At the point c, corresponding in Table II to the 

 temperature 2i72i, the cryohydric point has been reached 

 on a rising gradient, but no melting has taken place. Melting 

 is supposed to begin only when the temperature has reached 



2i72 exactly. Then there is a contraction of 0*458 c.c. 

 with no change of temperature, so that at this stage the 



coefficient of dilatation is = oc . When the tempe- 

 rature rises above 2i72, the apparent volume / increases 

 with the temperature, but at a gradually diminishing rate 

 until, at 7'O, the increase of volume due to simple expan- 

 sion of the ice is exactly balanced by the contraction due 

 to induced melting. At this temperature the coefficient of 



